Brett Romero

Data Inspired Insights

Tag: python

Official Release: Visual Analytics

I am proud to announce the release of an application I’ve been working on for the last few months – Visual Analytics. This application is designed to give you a new way to view your Google Analytics data using a range of interactive visualizations, allowing you to get a better understanding of who your users are, how they are getting to your site, and what they are doing when they get there.

For those worried about privacy and personal security, the application has a couple of features that will hopefully ease your mind. Firstly, there is no separate account or login details needed for Visual Analytics, everything is based on your existing Google account, and the login process is completed using Google authentication.

 

 

Secondly, the application does not currently store any user data. In fact, the application has no database at all (sensing a theme here?). That means that not only does that mean I can not sell your data to third parties, but that even if someone does manage to hack into the application, there is nothing to steal except my hacky code base.

For those interested in the technical specs, the backend of the application was built using Python and the Flask web framework. To access the data, once you are logged in using your Google credentials, the application makes calls to the Google Analytics API and then uses Pandas to handle the data manipulation (where needed). On the front end, the visualizations are created using D3.js and Highcharts (a big shout out to the Highcharts team and Mike Bostock for their excellent work on these libraries).

Anyway, if you have a Google Analytics account and are interested in getting some interesting insights into your data, take a look and let me know what you think. And please, if you find an issue or a bug, let me know!

 

 

How to create a flashcard app without a database

Last week, I covered how setting up a database may not be necessary when creating an app or visualization, even one that relies on data. This week we are going to walk through an example application that runs off data, but does not need a formal database.

First some background. For the last couple of months, I have been attending some basic Arabic classes to help get around Jordan easier. During a recent class, several of the students were discussing the time they had spent putting together physical cardboard flashcards to help them memorize words. Hearing this, and having played around with creating simple applications and visualizations for a couple of years now, it occurred to me that generating flashcards using a simple application would probably be significantly quicker and easier to do. In addition, if done the right way, it could work on your phone, making it available to you anytime you had your phone and an internet connection.

Perhaps as you are reading this, you are thinking that Arabic is a widely spoken language, surely there is already an app for that? And you would be correct, there are multiple apps for learning Arabic. However, the complication with Arabic is that each country/region uses a significantly different version of the language. In addition to these regional dialects, there is Modern Standard Arabic, which is the formal written version of the language. When it comes to the apps currently available, the version of Arabic being presented is almost always Modern Standard Arabic, as opposed to the Levantine Arabic which is spoken throughout Palestine, Jordan, Lebanon and Syria. Additionally, the apps are quite expensive (up to $10), of questionable accuracy/quality, or both.

To address this problem, and for the challenge and learning opportunity, a few weeks back I sat down over a weekend and put together a simple application that would generate Arabic flashcards (you can see the current version here). The application is based on an Excel spreadsheet with translations that I continue to enter over time based on my notes and the class textbook. Using an Excel spreadsheet in this case is advantageous for two key reasons:

  • It is simple to edit and update with new translations
  • It is a format that almost everyone is familiar with so I can recruit other students and teachers to add translations

With that out of the way, let’s take a look at the high-level process for creating a flashcards app.

1. Collecting the Data

The first step is creating the Excel spreadsheet for our translations. In this case, it is fairly simple and looks something like this:

arabic flashcards

In this spreadsheet, each row represents one word and, through the application, will represent one flashcard. In one column, we have the Arabic script for each word, then in the next column the English translation. In addition, we also have a third version of the word with a column header of ‘transcribed’. This column represents how the Arabic word would look/sound if written in Latin script, something that is commonly used in beginner classes when students cannot yet read Arabic script.[1] Finally, in the last column we have a category column. This will be used to provide a feature where the user can filter the words in the application, allowing them to focus their study on particular sets of words.

A quick note, we also have an ID column, which is not used in the application. It is only included to provide a unique key in our datasets, as good practice.

2. Processing the Data

The next step is to take the data from our spreadsheet, convert it to a format that we can use to generate flashcards in the application, then save it. To do this we will use my favorite Python library, pandas, and the short script shown below.

# -*- coding: utf-8 -*-
import pandas as pd

# Read In Data
df = pd.read_excel("./data.xlsx", header=0)

# Create JSON String
json_string = df.to_json(orient="records", force_ascii=False)
json_string = "var data = " + json_string + ";"

# Write to file
text_file = open("data.js", "w")
text_file.write(json_string)
text_file.close()

What this script in does is read in the file (in this case, data.xlsx) to a pandas dataframe (line 5). After that (line 8), we use the to_json method to output the contents of the dataframe to a JSON string. In line 9 we add some JavaScript to the beginning and end of that JSON string, then in lines 12-14 we save the string as a JavaScript file, data.js.

There are a couple of important things to note here. The first is that when dealing with non-Latin text characters (like Arabic characters), we need to specify that force_ascii=False (the default value is True). If we don’t do this, the script will return an error and/or convert the Arabic letters into a combination of Latin characters representing the Unicode character (i.e. it will look like gibberish).

The second thing to note for those that have not worked with JSON, or key-value stores more generally, is that this is the format that most data comes in when used in programs and applications. It is a highly flexible structure and, as a result, there are many ways we could represent the data shown above. In this case, we are using the ‘records’ format (as specified by pandas), which will look like this:

[

{
“id”:1,
“arabic”:”كتير”,
“english”:”A lot\/Many\/Very”,
“transcribed”:”kteer”,
“category”:”Basics”
},
{
“id”:2,
“arabic”:”عَن”,
“english”:”About”,
“transcribed”:”3an”,
“category”:”Basics”
},…

]

If this isn’t making any sense, or you would like to see some of the other possibilities, copy and paste some spreadsheet data into this CSV to JSON convertor. Toggling a few options, it should quickly become obvious how many different ways a given dataset can be represented in JSON format.

3. Building the App

Now that the data is ready, we create the files needed for the flashcards application. In this case, it is only three files, a HTML document (index.html) for the page, a CSS file for the styling, and an additional JavaScript file that will use the data in data.js to create the flashcards and generate the various features of the application. For those that are interested in the full code or want to create your own version, please feel free to checkout/fork the GitHub repo. For those that do not want to get too far into the weeds, there are just a few things I want to highlight about what the code is doing.

Firstly, the filtering and language options in the application are being generated directly from the data. What this means is that as more categories are added to the Excel spreadsheet, or if the languages change (i.e. the headings in the spreadsheet change), as soon as I update the underlying Excel and run the script shown above, all the options in the application will also update accordingly.

Secondly, I added a feature that allows the user to keep score. It is a simple honesty-based system, but I found it does provide some motivation to keep improving, as well as removing an element of self-deception as to how well you are actually doing. Often I would find myself thinking that I was getting almost all of them correct, only to find my correct percentage hovering around 70%.

Finally, a note on randomness. Whether the user is going through the cards unfiltered, or filtering for some category, the application is displaying the flashcards in a random[2] order. This random selection algorithm went through several iterations:

  1. In version 1, the algorithm would simply select four (the number of flashcards presented to the user at one time) random selections from the pool of eligible words.
  2. Upon testing version 1, it was found that, with surprising regularity, the same word would be selected more than once in a group of four flashcards. To address this, in version 2 a condition was added that when randomly selecting a word, it would only be accepted if that word had not already been selected in the given pool of four words.
  3. On further testing, I noticed another annoying issue. As I continually refreshed the four flashcards being displayed, some words would show up repeatedly, while others would take forever to show up, or not show up at all. To avoid this, for version 3, I changed the algorithm again. Now, instead of selecting four words at random, the algorithm instead took the whole list of words, shuffled them in a random order, and ran through the list in the new shuffled order. When the list ran out of words, it took the full list, shuffled it again, and continued.
  4. This was a big improvement. As I refreshed, I got different words, and was able to see all the words before they started repeating. But then I found another issue. In cases where the number of eligible words was not divisible by four, the old shuffled list and the new shuffled list would overlap in a selection of four words. In these cases, there was a possibility that the same word would be repeated. This is a little difficult to visualize, so the illustration below tries to present what was happening using an example list of ten words:

arabic flashcards

To address this, in version 4, a new condition was added. In cases like the example shown above, the algorithm will check the words from the new shuffled list to ensure they are not already selected from the old list. If a word is already selected, it will move that word to the end of the list and instead take the next word on the list. Here is another diagram to show what is happening:

arabic flashcards

4. Finishing Up

Ok, for those stepping through this and creating your own flashcards app, at this point you have copied the code available from the repo, made any changes to the spreadsheet, and rerun the script to refresh the data. For the final step, there are a couple of things that can be done.

If you are only planning to use the app on the same computer as you are using to create the flashcards app, you are done! Just open the index.html file using Chrome, Firefox or Safari (you can try Internet Explorer, but you know…) and you can test and use the app as you would use any website.

If you want to publish your flashcards app online to share with others, by far the easiest way is to use a service such as GitHub pages. I don’t want to turn this into a beginners guide to using git and GitHub, but there is excellent documentation available to help get you started if you would like to do this. You can see my version at the following address: https://vladimiriii.github.io/arabic-flashcards/, but there is even an option to redirect it to a domain of your choosing should you have one.

arabic flashcards

 

I hope this was a helpful guide to how a simple application can be created without a database, even if the application runs on some underlying form of data. Let me know what you think in the comments below!

 

[1] Because Arabic has many sounds that are difficult to convey in Latin script, this is also why when Arabic is transcribed, you will often find multiple spellings of the same word (e.g. Al-Qaeda vs Al-Qaida).

[2] As will be discussed in a new piece to be written, it is not truly random, and the reasons why are pretty interesting.

Forget SQL or NoSQL – 5 scenarios where you may not need a database at all

A while back, I attended a hackathon in Belgrade as a mentor. This hackathon was the first ‘open data’ hackathon in Serbia and focused on making applications using data that had recently been released by various ministries, government agencies, and independent bodies in Serbia. As we walked around talking to the various teams, one of the things I noticed at the time, was that almost all teams were using databases to manage their data . In most cases, the database being used was something very lightweight like SQLite3, but in some cases more serious databases (MySQL, PostgreSQL, MongoDB) were also being used.

What I have come to realize is that in many cases this was probably completely unnecessary, particularly given the tight timeframe the teams were working towards – a functional prototype within 48 hours. However, even if you have more time to build an application, there are several good reasons that you may not need to worry about using a formal database. These are outlined below.

1. The data is small

Firstly, let’s clarify what I mean when I say ‘small data’. For me, small data is any dataset under 10,000 records (assuming a reasonable number of data points for each record). For many non-data people, 10,000 records may seem quite big, but when using programming languages such as Python or JavaScript, this amount of data is usually very quick and easy to work with. In fact, as Josh Zeigler found, even loading 100,000 records or 15MB of data into a page was possible, completing in as little as 463ms (Safari FTW).

Leaving aside the numbers for a second, the key point here is that in many cases, the data being displayed in an application has far fewer than 10,000 records. If your data is less than 10,000 records, you should probably ask yourself, do you need a database? It is often far simpler, and requires significantly less overhead to simply have your data in a JSON file and load it into the page directly. Alternatively, CSV and Excel files can also be converted to JSON and dumped to a file very quickly and easily using a Python/Pandas script.

ecis visualization

The ECIS Development Tracker uses data from six Worldwide Governance Indicators and two other series over 20 years and 18 countries – a total of almost 3,000 data points and a perfect example of small data.

2. The data is static

Another reason you may not need a database is if you have a reasonable expectation that the data you are using is not going to change. This is often the case where the data is going to be used for read only purposes – for example visualizations, dashboards and other apps where you are presenting information to users. In these cases, again it may make sense to avoid a database, and rely on a flat file instead.

The important point here is that if the data is not changing or being altered, then static files are probably all that is needed. Even if the data is larger, you can use a script to handle any data processing and load the (assumedly) aggregated or filtered results into the page. If your needs are more dynamic (i.e. you want to show different data to different users and do not want to load everything), you may need a backend (something you would need for a database anyway) that extracts the required data from the flat file, but again, a database may be overkill.

kosovo mosaic

The Kosovo Mosaic visualizer – based on data from a survey conducted once every three years – is an example of a case where the data is not expected to change any time soon.

3. The data is simple

One of the big advantages of databases is their ability to store and provide access to complex data. For example, think about representing data from a chain of retail stores on the sale of various products by different sales people. In this case, because there are three related concepts (products, sales people and stores), representing this data without using a database becomes very difficult without a large amount of repetition[1]. In this case, even if the data is small and static, it may simply be better to use a relational database to store the data.

However, in cases where the data can be represented in a table, or multiple unrelated tables, subject to points 1 and 2 above, it may make sense to avoid the overhead of a database.

database schema

If you need a schema diagram like this to describe your data, you can probably skip the rest of this article.

4. The data is available from a good API

I have recently been working on a project to develop an application that is making extensive use of the Google API. While still under development, the app is already quite complex, making heavy use of data to generate charts and tables on almost every page. However, despite this complexity, so far, I have not had to use a database.

One of the primary reasons I have not needed to implement a database is that the Google API is flexible enough for me to effectively use that as a database. Every time I need data to generate a chart or table, the app makes a call to the API (using Python), passes the results to the front end where, because the data is small (the Google API returns a maximum of 10,000 rows in a query), most of the data manipulation is handled using JavaScript on the client side. For the cases where more heavy data manipulation is required, I make use of Python libraries like Pandas to handle the data processing before sending the data to the front end. What this boils down to is a data intensive application that, as yet, still does not need a database.

Of course, this isn’t to say I will not need a database in the future. If I plan to store user settings and preferences, track usage of the application, or collect other meta data, I will need to implement a database to store that information. However, if you are developing an application that will make use of a flexible and reliable API, you may not need to implement your own database.

google apis

Google has APIs available for almost all of its products – most of them with a lot of flexibility and quick response times.

5. The app is being built for a short-term need

While it might seem unusual to build a web app with the expectation that it will not be used six months later, this is a surprisingly common use case. In fact, this is often the expectation for visualizations and other informative pages, or pages built for a specific event.

In these particular use cases, keeping down overhead should be a big consideration, in addition to potential hosting options. Developing these short-term applications without a backend and database means free and easy hosting solutions like that provided by GitHub can be used. Adding a backend or database immediately means a more complex hosting setup is required.

Wrapping up, this is a not an argument against databases…

… it is simply an argument to use the best and simplest tools for a given job. As someone who has worked with a number of different databases throughout their career, I am actually a big user of databases and find most of them intuitive and easy to use. There is also a large number of advantages that only a database can provide, from ensuring data consistency, to facilitating large numbers of users simultaneously making updates, to managing large and complex datasets, there are a number of very good reasons to use a database (SQL or NoSQL, whichever flavor you happen to prefer).

But, as we have covered above, there may be some cases where you do not need these features and can avoid adding an unnecessary complication to your app.

 

Next week we’ll take a look at a simple app that uses an Excel spreadsheet to generate the data required for the application.

 

[1] With repetition comes an increased risk of data quality issues

JSONify It – CSV to JSON Converter

Go to JSONify It

For those who have some experience in creating visualizations, particularly online visualizations using JavaScript and libraries such as D3.js, one thing that you will often come across is the need to convert your data. Typically this need will arise because the data you receive or collect will be in a human-friendly format such as an Excel spreadsheet, and in order for you to use it for the visualization you will need that data in JSON format. Annoyingly, this will often be just a one time conversion, meaning writing a stand alone script to do the conversion often seems like overkill.

Handily, there are a number of CSV to JSON converters lying around on the internet for people to use, and most of them work more or less as expected. However, a problem I encountered when building this Procurement Zoomable Treemap visualization, is that you sometimes need the JSON to be nested, and this was not a feature I encountered on any of the online converters.

In order to address my need (and to see if I could pull it off), when I built that visualization I also used Python/Flask/Pandas to build a simple API that generated nested JSON datasets on the fly from an underlying CSV file. Having this allowed me to build the zoomable tree map that could be reconfigured by the user. That is, the user could specify the categories and the order in which the treemap would zoom through.

While this was great, it always felt a little incomplete. Then a few months back, I had some time on my hands and decided to take this API and upgrade it to have a full user interface so that, like many of the online convertors, users could copy and paste data straight into the browser, configure some options, and get a JSON formatted dataset back. The result was JSONify It – a simple but (I hope) easy to use app that is not only very flexible in formatting JSON, but as far as I can tell, is the only CSV to JSON convertor that allows you to nest the JSON by any column (or columns) you specify.

So, for those interested, feel free to take a look, try it out, look at the code, and if you come across any bugs or issues, or would like any further information, please let me know in the comments below.

 

Data Science: A Kaggle Walkthrough – Creating a Model

This article is Part VI in a series looking at data science and machine learning by walking through a Kaggle competition. If you have not done so already, you are strongly encouraged to go back and read the earlier parts – (Part I, Part II, Part III, Part IV and Part V).

Continuing on the walkthrough, in this part we build the model that will predict the first booking destination country for each user based on the dataset created in the earlier parts.

Choosing an Algorithm

The first step to building a model is to decide what type of algorithm to use. Below we look at some of the options.

Decision Tree

Arguably the most well known algorithm, and one of the simplest conceptually. The decision tree works in a similar manner to the decision tree that you might create when trying to understand which decision to make based on a range of variables.

The goal of the decision tree algorithm used for classification problems (like the one we are looking at) is to create one of these decision trees to classify records into a set number of categories. To do this, it starts with all the records in the training dataset and looks through all the features until it finds the one that allows it to most ‘cleanly’ split the records according to their categories. For example, if you are using daily weather data to try and determine whether it will rain the following day (i.e. there are two categories, ‘it does rain’ and ‘it does not rain’), the algorithm will look for a feature that best splits the records (in this case representing days) into those two categories. When it finds that feature, and the value to split on, it creates one point (‘decision node’) on the decision tree. It then takes each subpopulation and does the same thing again, building up a tree until either all the records are correctly classified, or the number in each subpopulation becomes too small to split. Below is an example decision tree using the described weather data to predict if it will rain tomorrow or not (thanks to Graham Williams’ excellent Rattle package for R):

The way to interpret the above tree is to start at the top. The first criteria the algorithm splits on is the humidity at 3pm. Starting with 100% of the records, if the the humidity at 3pm is less than 71, as it is the case for 93% of the records, we move to the left and find the next decision node. If the humidity at 3pm is greater than or equal to 71, we move to the right, which takes us to a leaf node where the model predicts that there will be rain tomorrow (‘yes’). We can see from the numbers in the node that this represents 7% of all records, and that 74% of the records that reach this node are correctly classified.

The first thing to note is that the model does not accurately predict whether it will rain tomorrow for all records, and in some leaf nodes, it is only slightly better than a coin toss. This is not necessarily a bad thing. The biggest problem that data scientists have with decision trees is the classic problem of overfitting. In the example above, parameters have been set to stop model splitting once the population of records at a given node gets too small (minimum split) and when a certain number of splits have occurred (‘maximum depth’). These values have been set at values to prevent the tree from growing to large. The reason for this is that if the tree gets too large, it will start modelling random noise and hence will not work for data not in the training dataset (it will not ‘generalize’ well).

To picture what this means, imagine extending the example decision tree above further until the model starts splitting out single records using criteria like ‘Humidty3pm = 54’ and ‘Humidty3pm = 31’. That type of decision node may work for this particular training data because there is a specific record that meet that criteria, but it is highly unlikely that it represents any predictive ability and so is unlikely to be accurate if applied to other data.

All this discussion of overfitting with decision trees does however raise an important problem. That problem is how do you know how large you should grow the tree. How do you set the parameters to avoid overfitting but still have an accurate model? The truth is that is is extremely difficult to know how to set the parameters. Set them too conservatively and the model will lose too much predictive power. Set them too aggressively and the model will start overfitting the data.

Seeing the Forest for the Trees

Given the limitations of decisions trees and the risk of overfitting, it may be tempting to think “why bother?” Fortunately, methods have been found to reduce the risk of overfitting and increase predictive power of decisions trees and the two most popular methods both have the same basic premise – to train multiple trees.

One of the most well known algorithms that utilizes decision trees is the ‘random forest’ algorithm. As the name suggests, the algorithm constructs a large number of different trees (as defined by the user) by randomly selecting the features that can be used to build each tree (as opposed to using all the features for each tree). Typically, the trees in a random forest also have the parameters set to ensure each tree will also be relatively shallow, meaning that the algorithm creates a large number of shallow decision trees (decision bonsai?). Once the trees are constructed, each tree is used to predict the outcome for a new record, with these multiple predictions then serving as votes, with a majority rules approach applied.

Another algorithm which has become almost the default algorithm of choice for Kagglers, and is the type of the model we will use, uses a method called ‘boosting’, which means it builds trees iteratively such that each tree ‘learns’ from earlier trees. To do this the algorithm builds a first tree – again typically a shallower tree than if you were going to use a one tree approach – and makes predictions using that tree. Then the algorithm finds the records that are misclassified by that tree, and assigns a higher weight of importance to those records than the records that were correctly classified. The algorithm then builds a new tree with these new weightings. This whole process is repeated as many times as specified by the user. Once the specified number of trees have been built, all the trees built during this process are used to classify the records, with a majority rules approach used to determine the final prediction.

It should be noted that this methodology (‘boosting’) can actually be applied to many classification algorithms, but has really grown popular with the decision tree based implementation. It should also be noted there are different implementations of this algorithm even just using trees. In this case, we will be using the very popular XGBoost algorithm.

Alternative Models

So far we have only covered decision trees and decision tree-based algorithms. However, there are a range of different algorithms that can be used for classification problems. Given this is supposed to be a short blog series, I will not go into too much detail on each algorithm here. But if you want more information on these algorithms, or other algorithms that I haven’t covered here, there is a growing amount of information online. I also strongly recommend the Data Science specialization offered by John Hopkins University, for free, on Coursera.

K-Nearest Neighbors

The K-nearest neighbor algorithms are arguably one of the simplest algorithms in concept. The algorithm classifies a given object by looking at the classification of the k most similar records[1] and seeing how those records are classified. This type of algorithm is called a lazy learner because during the training phase, it essentially just stores the data provided. Only when a new object needs to be classified does the algorithm start looking through the data to try to find the closest matches.

Neural Networks

As the name suggests, these algorithms simulate biological networks by creating a series of nodes and connecting them together. A neural network typically consists of three layers; an input layer, a hidden layer (although there can be multiple hidden layers) and an output layer.

A model is trained by passing records through the network and weights adjusted at each node continually adjusted to ensure that the record ends up at the right ‘output node’.

Support Vector Machines

This type of algorithm, commonly used for text classification problems, is arguably the most difficult to visualize. At the simplest level, the algorithm tries to draw straight lines (or planes for classifications with more than 2 features) that best separate the classes provided. Although this sounds like a fairly simplistic approach to classifying objects, it becomes far more powerful due to the transformations (sometimes called a ‘kernel trick’) the algorithm can apply to the data before drawing these lines/planes. The mathematics behind this are far too complex to go into here, but the Wikipedia page has some nice visuals to help picture how this is working. In addition, this video provides a nice example of how a Support Vector Machine can separate classes using this kernel trick:

Creating the Model

Back to the modelling – now that we know what algorithm we are using (XGBoost algorithm for those skipping ahead), let talk about the approach.

Cross Validation

As mentioned in regards to decision trees, one of the keys risks when creating models of any type is the risk of overfitting. One of the primary ways data scientists will guard against overfitting is to estimate the accuracy of their models on data that was not used to train the model. To do this they typically use a method called cross validation. There are different methods for doing cross validation, but the method we will employ is called k-fold cross validation.

k-fold cross validation involves splitting the training data into k subsets (where k is greater than or equal to 2), training the model using k – 1 of those subsets, then running the model on the subset that was not used in the training process. Because all of the data used in the cross validation process is training data, the correct classification for each record is known and so the predicted category can be compared to the actual category. Once all folds have been completed, the average score across all folds is taken as an estimate of how the model will perform on other data. An example of a 3-fold cross validation is shown below:

Parameter Tuning

As you may have realized from the earlier description of the XGBoost algorithm – there are quite a few options (parameters) that we need to define to build the model. How many trees to build? How deep should each tree be? How much extra weight will be attached to each misclassified record? Tuning these parameters to get the best results from the model is often one of the most time consuming things that data scientists do. Fortunately, the process can be automated to a large degree so that we do not have to sit there rerunning the model repeatedly and noting down the results. Even better, using the Scikit-Learn package, we can merge the parameter tuning and cross validation steps into one, allowing us to search for the best combination of parameters while using k-fold cross validation to verify the results.

Training the Model

In order to train the model (using cross validation and parameter tuning as outlined above), we first need to define our training dataset – remembering that we previously combined the training and test data to simplify the cleaning and transforming process. To feed these into the model, we also need to split the training data into the three main components – the user IDs (we don’t want to use these for training as they are randomly generated), the features to use for training (X), and the categories we are trying to predict (y).

# Prepare training data for modelling
df_train.set_index('id', inplace=True)
df_train = pd.concat([df_train['country_destination'], df_all], axis=1, join='inner')

id_train = df_train.index.values
labels = df_train['country_destination']
le = LabelEncoder()
y = le.fit_transform(labels)
X = df_train.drop('country_destination', axis=1, inplace=False)

Now that we have our training data ready, we can use GridSearchCV to run the algorithm with a range of parameters, then select the model that has the highest cross validated score based on the chosen measure of a performance (in this case accuracy, but there are a range of metrics we could use based on our needs).

# Grid Search - Used to find best combination of parameters
XGB_model = xgb.XGBClassifier(objective='multi:softprob', subsample=0.5, colsample_bytree=0.5, seed=0)
param_grid = {'max_depth': [3, 4, 5], 'learning_rate': [0.1, 0.3], 'n_estimators': [25, 50]}
model = grid_search.GridSearchCV(estimator=XGB_model, param_grid=param_grid, scoring='accuracy', verbose=10, n_jobs=1, iid=True, refit=True, cv=3)

model.fit(X, y)
print("Best score: %0.3f" % model.best_score_)
print("Best parameters set:")
best_parameters = model.best_estimator_.get_params()
for param_name in sorted(param_grid.keys()):
print("\t%s: %r" % (param_name, best_parameters[param_name]))

Please note that running this step can take a significant amount of time. Running the algorithm with 25 trees takes around 2.5 minutes for each cross validation on my Macbook Pro. Running the script above with all the options specified will likely take well over an hour.

Making the Predictions

Now that we have trained a model based on the best parameters, the next step is to use the model to make predictions for the records in the testing dataset. Again we need to extract the testing data out of the combined dataset we created for the cleaning and transformation steps, and again we need to separate the main components for the model. After these steps, we use the model created in the previous step to make the predictions.

# Prepare test data for prediction
df_test.set_index('id', inplace=True)
df_test = pd.merge(df_test.loc[:,['date_first_booking']], df_all, how='left', left_index=True, right_index=True, sort=False)
X_test = df_test.drop('date_first_booking', axis=1, inplace=False)
X_test = X_test.fillna(-1)
id_test = df_test.index.values

# Make predictions
y_pred = model.predict_proba(X_test)

As you may have noted from the code above, we have used the predict_proba method instead of the usual predict method. This is done because of the way Kaggle will assess the results for this particular competition. Rather than just assessing one prediction for each user, Kaggle will assess up to 5 predictions for each user. In order to maximize the score, we will use the predicted probabilities that predict_proba produces to select the 5 best predictions. Finally, we will write these results to a file that will be created in the same folder as the script.

#Taking the 5 classes with highest probabilities
ids = []  #list of ids
cts = []  #list of countries
for i in range(len(id_test)):
idx = id_test[i]
ids += [idx] * 5
cts += le.inverse_transform(np.argsort(y_pred[i])[::-1])[:5].tolist()

#Generate submission
print("Outputting final results...")
sub = pd.DataFrame(np.column_stack((ids, cts)), columns=['id', 'country'])
sub.to_csv('./submission.csv', index=False)

For those that wish to, you should be able to submit the file produced from this script on Kaggle. The competition is now finished and you will not receive an official position on the leaderboard, but your results will be processed and you will be told where you would have finished.

Wrapping Up

Those that are more experienced with data science may realize this series, as lengthy as it is, does not even scratch the surface of a lot of topics related to data science. Unsupervised learning, association rules mining, text analytics and deep learning are all topics that have not been covered at all. Unfortunately, the full scope of data science and machine learning are not something that can be covered in a blog. That said, I did have two goals for those reading these blog articles.

Firstly, I hope that this series demystifies some aspects of data science for those that currently see it as a black box. Although one can spend their career working in data science and still not master all aspects, even a cursory understanding of how machine learning algorithms work can help provide understanding as to what sort of questions machine learning can help to answer, and what sort of questions are problematic.

Secondly, I hope this series encourages some of you to dig deeper, to learn more about this topic. Machine learning is a rapidly growing field that is expanding to every aspect of life. This includes, recommendation engines on websites, astronomy – where it helps to identify stars and planets, the pharmaceutical industry – where it is being used to predict which molecular structures that are likely to produce useful drugs, and maybe most famously, in training self‑driving cars to drive in the real world. Whatever your primary interest, there is likely to be some machine learning applications being developed or being used already.

[1] There are a range of metrics that can be used to do this. For available metrics in the Scikit Learn package, see here.

Full script:

import pandas as pd
import numpy as np
import xgboost as xgb

from sklearn import cross_validation, decomposition, grid_search
from sklearn.preprocessing import LabelEncoder

####################################################
# Functions #
####################################################
# Remove outliers
def remove_outliers(df, column, min_val, max_val):
col_values = df[column].values
df[column] = np.where(np.logical_or(col_values<=min_val, col_values>=max_val), np.NaN, col_values)

return df

# Home made One Hot Encoder
def convert_to_binary(df, column_to_convert):
categories = list(df[column_to_convert].drop_duplicates())

for category in categories:
cat_name = str(category).replace(" ", "_").replace("(", "").replace(")", "").replace("/", "_").replace("-", "").lower()
col_name = column_to_convert[:5] + '_' + cat_name[:10]
df[col_name] = 0
df.loc[(df[column_to_convert] == category), col_name] = 1

return df

# Count occurrences of value in a column
def convert_to_counts(df, id_col, column_to_convert):
id_list = df[id_col].drop_duplicates()

df_counts = df.loc[:,[id_col, column_to_convert]]
df_counts['count'] = 1
df_counts = df_counts.groupby(by=[id_col, column_to_convert], as_index=False, sort=False).sum()

new_df = df_counts.pivot(index=id_col, columns=column_to_convert, values='count')
new_df = new_df.fillna(0)

# Rename Columns
categories = list(df[column_to_convert].drop_duplicates())
for category in categories:
cat_name = str(category).replace(" ", "_").replace("(", "").replace(")", "").replace("/", "_").replace("-", "").lower()
col_name = column_to_convert + '_' + cat_name
new_df.rename(columns = {category:col_name}, inplace=True)

return new_df

####################################################
# Cleaning #
####################################################
# Import data
print("Reading in data...")
tr_filepath = "./train_users_2.csv"
df_train = pd.read_csv(tr_filepath, header=0, index_col=None)
te_filepath = "./test_users.csv"
df_test = pd.read_csv(te_filepath, header=0, index_col=None)

# Combine into one dataset
df_all = pd.concat((df_train, df_test), axis=0, ignore_index=True)

# Change Dates to consistent format
print("Fixing timestamps...")
df_all['date_account_created'] = pd.to_datetime(df_all['date_account_created'], format='%Y-%m-%d')
df_all['timestamp_first_active'] = pd.to_datetime(df_all['timestamp_first_active'], format='%Y%m%d%H%M%S')
df_all['date_account_created'].fillna(df_all.timestamp_first_active, inplace=True)

# Remove date_first_booking column
df_all.drop('date_first_booking', axis=1, inplace=True)

# Fixing age column
print("Fixing age column...")
df_all = remove_outliers(df=df_all, column='age', min_val=15, max_val=90)
df_all['age'].fillna(-1, inplace=True)

# Fill first_affiliate_tracked column
print("Filling first_affiliate_tracked column...")
df_all['first_affiliate_tracked'].fillna(-1, inplace=True)

####################################################
# Data Transformation #
####################################################
# One Hot Encoding
print("One Hot Encoding categorical data...")
columns_to_convert = ['gender', 'signup_method', 'signup_flow', 'language', 'affiliate_channel', 'affiliate_provider', 'first_affiliate_tracked', 'signup_app', 'first_device_type', 'first_browser']

for column in columns_to_convert:
df_all = convert_to_binary(df=df_all, column_to_convert=column)
df_all.drop(column, axis=1, inplace=True)

####################################################
# Feature Extraction #
####################################################
# Add new date related fields
print("Adding new fields...")
df_all['day_account_created'] = df_all['date_account_created'].dt.weekday
df_all['month_account_created'] = df_all['date_account_created'].dt.month
df_all['quarter_account_created'] = df_all['date_account_created'].dt.quarter
df_all['year_account_created'] = df_all['date_account_created'].dt.year
df_all['hour_first_active'] = df_all['timestamp_first_active'].dt.hour
df_all['day_first_active'] = df_all['timestamp_first_active'].dt.weekday
df_all['month_first_active'] = df_all['timestamp_first_active'].dt.month
df_all['quarter_first_active'] = df_all['timestamp_first_active'].dt.quarter
df_all['year_first_active'] = df_all['timestamp_first_active'].dt.year
df_all['created_less_active'] = (df_all['date_account_created'] - df_all['timestamp_first_active']).dt.days

# Drop unnecessary columns
columns_to_drop = ['date_account_created', 'timestamp_first_active', 'date_first_booking', 'country_destination']
for column in columns_to_drop:
if column in df_all.columns:
df_all.drop(column, axis=1, inplace=True)

####################################################
# Add data from sessions.csv #
####################################################
# Import sessions data
s_filepath = "./sessions.csv"
sessions = pd.read_csv(s_filepath, header=0, index_col=False)

# Determine primary device
print("Determing primary device...")
sessions_device = sessions.loc[:, ['user_id', 'device_type', 'secs_elapsed']]
aggregated_lvl1 = sessions_device.groupby(['user_id', 'device_type'], as_index=False, sort=False).aggregate(np.sum)
idx = aggregated_lvl1.groupby(['user_id'], sort=False)['secs_elapsed'].transform(max) == aggregated_lvl1['secs_elapsed']
df_primary = pd.DataFrame(aggregated_lvl1.loc[idx , ['user_id', 'device_type', 'secs_elapsed']])
df_primary.rename(columns = {'device_type':'primary_device', 'secs_elapsed':'primary_secs'}, inplace=True)
df_primary = convert_to_binary(df=df_primary, column_to_convert='primary_device')
df_primary.drop('primary_device', axis=1, inplace=True)

# Determine Secondary device
print("Determing secondary device...")
remaining = aggregated_lvl1.drop(aggregated_lvl1.index[idx])
idx = remaining.groupby(['user_id'], sort=False)['secs_elapsed'].transform(max) == remaining['secs_elapsed']
df_secondary = pd.DataFrame(remaining.loc[idx , ['user_id', 'device_type', 'secs_elapsed']])
df_secondary.rename(columns = {'device_type':'secondary_device', 'secs_elapsed':'secondary_secs'}, inplace=True)
df_secondary = convert_to_binary(df=df_secondary, column_to_convert='secondary_device')
df_secondary.drop('secondary_device', axis=1, inplace=True)

# Aggregate and combine actions taken columns
print("Aggregating actions taken...")
session_actions = sessions.loc[:,['user_id', 'action', 'action_type', 'action_detail']]
columns_to_convert = ['action', 'action_type', 'action_detail']
session_actions = session_actions.fillna('not provided')
first = True

for column in columns_to_convert:
print("Converting " + column + " column...")
current_data = convert_to_counts(df=session_actions, id_col='user_id', column_to_convert=column)

# If first loop, current data becomes existing data, otherwise merge existing and current
if first:
first = False
actions_data = current_data
else:
actions_data = pd.concat([actions_data, current_data], axis=1, join='inner')

# Merge device datasets
print("Combining results...")
df_primary.set_index('user_id', inplace=True)
df_secondary.set_index('user_id', inplace=True)
device_data = pd.concat([df_primary, df_secondary], axis=1, join="outer")

# Merge device and actions datasets
combined_results = pd.concat([device_data, actions_data], axis=1, join='outer')
df_sessions = combined_results.fillna(0)

# Merge user and session datasets
df_all.set_index('id', inplace=True)
df_all = pd.concat([df_all, df_sessions], axis=1, join='inner')

####################################################
# Building Model #
####################################################
# Prepare training data for modelling
df_train.set_index('id', inplace=True)
df_train = pd.concat([df_train['country_destination'], df_all], axis=1, join='inner')

id_train = df_train.index.values
labels = df_train['country_destination']
le = LabelEncoder()
y = le.fit_transform(labels)
X = df_train.drop('country_destination', axis=1, inplace=False)

# Training model
print("Training model...")

# Grid Search - Used to find best combination of parameters
XGB_model = xgb.XGBClassifier(objective='multi:softprob', subsample=0.5, colsample_bytree=0.5, seed=0)
param_grid = {'max_depth': [3, 4], 'learning_rate': [0.1, 0.3], 'n_estimators': [25, 50]}
model = grid_search.GridSearchCV(estimator=XGB_model, param_grid=param_grid, scoring='accuracy', verbose=10, n_jobs=1, iid=True, refit=True, cv=3)

model.fit(X, y)
print("Best score: %0.3f" % model.best_score_)
print("Best parameters set:")
best_parameters = model.best_estimator_.get_params()
for param_name in sorted(param_grid.keys()):
print("\t%s: %r" % (param_name, best_parameters[param_name]))

####################################################
# Make predictions #
####################################################
print("Making predictions...")

# Prepare test data for prediction
df_test.set_index('id', inplace=True)
df_test = pd.merge(df_test.loc[:,['date_first_booking']], df_all, how='left', left_index=True, right_index=True, sort=False)
X_test = df_test.drop('date_first_booking', axis=1, inplace=False)
X_test = X_test.fillna(-1)
id_test = df_test.index.values

# Make predictions
y_pred = model.predict_proba(X_test)

#Taking the 5 classes with highest probabilities
ids = [] #list of ids
cts = [] #list of countries
for i in range(len(id_test)):
idx = id_test[i]
ids += [idx] * 5
cts += le.inverse_transform(np.argsort(y_pred[i])[::-1])[:5].tolist()

#Generate submission
print("Outputting final results...")
sub = pd.DataFrame(np.column_stack((ids, cts)), columns=['id', 'country'])
sub.to_csv('./submission.csv',index=False)

Data Science: A Kaggle Walkthrough – Adding New Data

This article is Part V in a series looking at data science and machine learning by walking through a Kaggle competition. If you have not done so already, you are strongly encouraged to go back and read the earlier parts – (Part I, Part II, Part III and Part IV).

Continuing on the walkthrough, in this part we take the data from sessions.csv that we left aside initially and add it to the transformed and expanded data from Part IV.  This part will cover, in brief, all the steps in Parts II – IV.

Understanding the Data

As we did for the user data in training.csv, the first step here is to understand what the data in sessions.csv looks like. Although this file, with over 10 million rows, is too large to display in entirety in Excel[1], we can still open the file using Excel to get an understanding of what columns we have and what at least the first million rows of data looks like. Some sample rows are provided below:

user_idactionaction_typeaction_detaildevice_typesecs_elapsed
4rvqpxoh3hcampaigns-unknown--unknown-iPhone375
4rvqpxoh3hactive-unknown--unknown-iPhone728
4rvqpxoh3hcreate-unknown--unknown-iPhone
4rvqpxoh3hnotifications-unknown--unknown-iPhone187
4rvqpxoh3hlistings-unknown--unknown-iPhone154
4rvqpxoh3hunavailabilities-unknown--unknown-iPhone204
4rvqpxoh3hindex-unknown--unknown-iPhone21
4rvqpxoh3hindex-unknown--unknown-iPhone886
c8mfesvkv0confirm_emailclickconfirm_email_linkiPad Tablet1371616
c8mfesvkv0header_userpicdataheader_userpiciPad Tablet8672
c8mfesvkv0createsubmitcreate_useriPad Tablet
xwxei6hdk4dashboardviewdashboardiPhone1355
xwxei6hdk4header_userpicdataheader_userpiciPhone1246
xwxei6hdk4message_postmessage_postiPad Tablet
xwxei6hdk4ask_questionsubmitcontact_hostiPad Tablet386
xwxei6hdk4ask_questionsubmitcontact_hostiPad Tablet424
xwxei6hdk4message_postmessage_postiPad Tablet0
xwxei6hdk4confirm_emailclickconfirm_email_linkiPhone46262

As can be seen, the dataset contains records of user actions, with each row representing one action a user took. Every time a user reviewed search results, updated a wish list or updated their account information, a new row was created in this dataset. Although this data is likely to be very useful for our goal of predicting which country a user will make their first booking in, it also complicates the process of combining this data with the data from training.csv, as it will have to be aggregated so that there is one row per user (as opposed to many rows for each user, currently).

Aside from details of the actions taken, there are a couple of interesting fields in this data. The first is device_type – this field contains the type of device used for the specified action. The second interesting field is the secs_elapsed field. This shows us how long (in seconds) was spent on a particular action.

Both of these fields provide us with potentially important information that could help to more accurately predict which country a user will make a first booking in. For example, it is not difficult to imagine that people spending relatively little time to make a booking on a phone are likely to be making bookings in locations closer to home (i.e. the US) than someone spending more time to make a booking on a desktop computer. Of course this is just a theory that needs to be proven, but it is a good reason to ensure we are capturing this information in our final training dataset.

Cleaning and Transforming the Data

Now that we have a basic understanding of the data, we need to undertake the cleaning and transformation steps. Because of the structure of this data (and for the sake of brevity), we are going to do both of these things at the same time.

The first step is to import the data:

# Import sessions data
s_filepath = "./sessions.csv"
sessions = pd.read_csv(s_filepath, header=0, index_col=False)

Extract the primary and secondary devices for each user

Remembering that we need to get the final data into a format that can be merged with the data created in Part IV (i.e. a dataset where one row equals one user), the first piece of information we are going to extract is the primary and secondary device for each user. How do we determine what a user’s primary and secondary devices are? We look at how much time they spent on each device. In short we are going to make the following changes to the data:

devices

One thing to note as we make these transformations is that by aggregating the data this way, we are also implicitly removing the missing values. The code to do this transformation is shown below:

# Determine primary device
print("Determing primary device...")
sessions_device = sessions.loc[:, ['user_id', 'device_type', 'secs_elapsed']]
aggregated_lvl1 = sessions_device.groupby(['user_id', 'device_type'], as_index=False, sort=False).aggregate(np.sum)
idx = aggregated_lvl1.groupby(['user_id'], sort=False)['secs_elapsed'].transform(max) == aggregated_lvl1['secs_elapsed']
df_primary = pd.DataFrame(aggregated_lvl1.loc[idx , ['user_id', 'device_type', 'secs_elapsed']])
df_primary.rename(columns = {'device_type':'primary_device', 'secs_elapsed':'primary_secs'}, inplace=True)
df_primary = convert_to_binary(df=df_primary, column_to_convert='primary_device')
df_primary.drop('primary_device', axis=1, inplace=True)

# Determine Secondary device
print("Determing secondary device...")
remaining = aggregated_lvl1.drop(aggregated_lvl1.index[idx])
idx = remaining.groupby(['user_id'], sort=False)['secs_elapsed'].transform(max) == remaining['secs_elapsed']
df_secondary = pd.DataFrame(remaining.loc[idx , ['user_id', 'device_type', 'secs_elapsed']])
df_secondary.rename(columns = {'device_type':'secondary_device', 'secs_elapsed':'secondary_secs'}, inplace=True)
df_secondary = convert_to_binary(df=df_secondary, column_to_convert='secondary_device')
df_secondary.drop('secondary_device', axis=1, inplace=True)

Determine Counts of Actions

The next thing we are going to do is take counts of how many times each action was taken by each user. This is a two-step process. The first step is to determine the count of each action type for each user:

Step 1

actions_step_1

Step 2

actions_step_2

For you Excel buffs out there, the second step might strike you as something that could be achieved using a pivot table – and you would be right. In fact, the custom function that we use to make this transformation uses a pandas method called ‘pivot’. This is important to note for a couple of reasons. The first is that, with all the talk about new data, people who have worked with data mostly (or entirely) using ‘old technology’ like Excel and SQL are often given the impression that their skills are redundant or not useful in modern data science. As this example shows, the ways of thinking about data that you develop working with Excel and SQL are not only relevant, but often extremely useful.

The second reason is that for people (like me) who do not know all the methods available for pandas dataframes off by heart, being able to identify techniques you have used in other programs and languages provides you with a way to find corresponding methods in new languages. I discovered this method by searching for “pandas pivot”, knowing that this way of manipulating data was likely to have some equivalent in pandas.

Looping Through the Actions Columns

Looking at the examples above, you may have realized that the transformation as shown only works for one action column at a time, but in the data we have three action columns: action, action_type and action_detail.

To handle the multiple action columns, we repeat these steps for each column individually, effectively creating three separate tables. Because we have now created tables where each row represents one user, we can now join (another concept SQL users will be very familiar with) these three tables together on the basis of the user id. The full code for these steps is shown below:

# Count occurrences of value in a column
def convert_to_counts(df, id_col, column_to_convert):
id_list = df[id_col].drop_duplicates()

df_counts = df.loc[:,[id_col, column_to_convert]]
df_counts['count'] = 1
df_counts = df_counts.groupby(by=[id_col, column_to_convert], as_index=False, sort=False).sum()

new_df = df_counts.pivot(index=id_col, columns=column_to_convert, values='count')
new_df = new_df.fillna(0)

# Rename Columns
categories = list(df[column_to_convert].drop_duplicates())
for category in categories:
cat_name = str(category).replace(" ", "_").replace("(", "").replace(")", "").replace("/", "_").replace("-", "").lower()
col_name = column_to_convert + '_' + cat_name
new_df.rename(columns = {category:col_name}, inplace=True)

return new_df

# Aggregate and combine actions taken columns
print("Aggregating actions taken...")
session_actions = sessions.loc[:,['user_id', 'action', 'action_type', 'action_detail']]
columns_to_convert = ['action', 'action_type', 'action_detail']
session_actions = session_actions.fillna('not provided')
first = True

for column in columns_to_convert:
print("Converting " + column + " column...")
current_data = convert_to_counts(df=session_actions, id_col='user_id', column_to_convert=column)

# If first loop, current data becomes existing data, otherwise merge existing and current
if first:
first = False
actions_data = current_data
else:
actions_data = pd.concat([actions_data, current_data], axis=1, join='inner')

Combine Data Sets

The final steps are to combine the various datasets we have created into one large dataset. First we combine the two device dataframes (df_primary and df_secondary) to create a device dataframe. Then we combine the device dataframe with the actions dataframe to create a sessions dataframe with all the features we extracted from sessions.csv. Finally, we combine the sessions dataframe with the user data dataframe from Part IV. The code for the various combinations is shown below:

# Merge device datasets
print("Combining results...")
df_primary.set_index('user_id', inplace=True)
df_secondary.set_index('user_id', inplace=True)
device_data = pd.concat([df_primary, df_secondary], axis=1, join="outer")

# Merge device and actions datasets
combined_results = pd.concat([device_data, actions_data], axis=1, join='outer')
df_sessions = combined_results.fillna(0)

# Merge user and session datasets
df_all.set_index('id', inplace=True)
df_all = pd.concat([df_all, df_sessions], axis=1, join='inner')

A Note on Joins

For those that can read a little bit of code and are familiar with joins in SQL, you may be asking why I am using (full) outer joins for the first two combinations, but an inner join for the final step[2].

The first step requires an outer join because not all users have a secondary device. That is, some users only logged onto Airbnb using one device (or at least one type of device). Doing an outer join here ensures that our dataset includes all users regardless of this fact.

The second step could use an inner or an outer join, as both the device and actions datasets should contain all users. In this case we use an outer join just to ensure that if a user is missing from one of the datasets (for whatever reason), we will still capture them. You may also notice that after the second step we fill any missing values with 0s to ensure we do not have any NULL values that may have been generated by these outer joins.

For the third step we use an inner join for a key reason – we want our final training dataset to only include users that also have sessions data. Using an inner join here is an easy way to join the datasets and filter for the users with sessions data in one step.

Wrapping Up

In the first four parts of this series, we looked in detail at some of the various steps in the process of building a model. Although these steps should be distinct thought processes that occur for each model building process, hopefully what this article provides is an insight into how some of these steps can be combined if planned out carefully. In relatively few steps, we have taken a dataset containing 10 million rows of user actions data, cleaned it, extracted a bunch of important information, and added it to our user data, ready for training a model.

The other important thing to take away from this article is how useful ‘old school’ ways of thinking about data still are. For all the talk about unstructured data and NoSQL databases, the fact is that knowing how to work with and manipulate old fashioned columns and rows is still as important as ever. Whether it is joins and aggregation in SQL, pivot tables and VLOOKUPS in Excel, or just the general concept of relational data, not only is that knowledge relevant, but it is often extremely useful.

Next Time

In the next piece, we will finally get to the good stuff and train the algorithm to make the final predictions.

 

[1] Nope, still doesn’t qualify as ‘Big Data’…

[2] For those that do not understand what I mean by inner and outer joins (and are interested in knowing) – stackoverflow comes to the rescue again with this great illustrated answer.

Data Science: A Kaggle Walkthrough – Introduction

I have spent a lot of time working with spreadsheets, databases, and data more generally. This work has led to me having a very particular set of skills, skills I have acquired over a very long career. Skills that make me a nightmare for people like you. If you let my daughter go now, that’ll be the end of it. I will not look for you, I will not pursue you. But if you don’t, I will look for you, I will find you, and I will kill you.

The badassery of Liam Neeson aside, although I have spent years working with data in a range of capacities, the skills and techniques required for ‘data science’ are a very specific subset that do not tend to come up in too many jobs. What is more, data science tends to involve a lot more programming than most other data related work and this can be intimidating for people who are not coming from a computer science background. The problem is, people who work with data in other contexts (e.g. economics and statistics), as well as those with industry specific experience and knowledge, can often bring different and important perspectives to data science problems. Yet, these people often feel unable to contribute because they do not understand programming or the black box models being used.

Something that has nothing to do with data science

Therefore, in a probably futile attempt to shed some light on this field, this will be the first part in a multi-part series looking at what data science involves and some of the techniques most commonly used. This series is not intended to make everyone experts on data science, rather it is intended to simply try and remove some of the fear and mystery surrounding the field. In order to be as practical as possible, this series will be structured as a walk through of the process of entering a Kaggle competition and the steps taken to arrive at the final submission.

What is Kaggle?

For those that do not know, Kaggle is a website that hosts data science problems for an online community of data science enthusiasts to solve. These problems can be anything from predicting cancer based on patient data, to sentiment analysis of movie reviews and handwriting recognition – the only thing they all have in common is that they are problems requiring the application of data science to be solved.

The problems on Kaggle come from a range of sources. Some are provided just for fun and/or educational purposes, but many are provided by companies that have genuine problems they are trying to solve. As an incentive for Kaggle users to compete, prizes are often awarded for winning these competitions, or finishing in the top x positions. Sometimes the prize is a job or products from the company, but there can also be substantial monetary prizes. Home Depot for example is currently offering $40,000 for the algorithm that returns the most relevant search results on homedepot.com.

Despite the large prizes on offer though, many people on Kaggle compete simply for practice and the experience. The competitions involve interesting problems and there are plenty of users who submit their scripts publically, providing an excellent opportunity for learning for those just trying to break into the field. There are also active discussion forums full of people willing to provide advice and assistance to other users.

What is not spelled out on the website, but is assumed knowledge, is that to make accurate predictions, you will have to use machine learning.

Machine Learning

When it comes to machine learning, there is a lot of general misunderstanding about what this actually involves. While there are different forms of machine learning, the one that I will focus on here is known as classification, which is a form of ‘supervised learning’. Classification is the process of assigning records or instances (think rows in a dataset) to a specific category in a pre-determined set of categories. Think about a problem like predicting which passengers on the Titanic survived (i.e. there are two categories – ‘survived’ and ‘did not survive’) based on their age, class and gender[1].

Titanic Classification Problem

[table “titanic_example” not found /]

Referring specifically to ‘supervised learning’ algorithms, the way these predictions are made is by providing the algorithm with a dataset (typically the larger the better) of ‘training data’. This training data contains all the information available to make the prediction as well as the categories each record corresponds to. This data is then used to ‘train’ the algorithm to find the most accurate way to classify those records for which we do not know the category.

Training Data

PassengerAgeClassGenderSurvived?
001323SecondFemale1
001421SteerageFemale0
001546SteerageMale0
001632FirstMale0
001713FirstFemale1
001824SecondMale0
001929FirstMale1
002080SecondMale1
00219SteerageFemale0
002244SteerageMale0
002335SteerageFemale1
002410SteerageMale0

Although that seems relatively straightforward, part of what makes data science such a complex field is the limitless number of ways that a predictive model can be built. There are a huge number of different algorithms that can be trained, mostly with weird sounding names like Neural Network, Random Forest and Support Vector Machine (we will look at some of these in more detail in future installments). These algorithms can also be combined to create a single model. In fact, the people/teams that end up winning Kaggle competitions often combine the predictions of a number of different algorithms.

To make things more complicated, within each algorithm, there is a range of parameters that can be adjusted to significantly alter the prediction accuracy, and these parameters will vary for each classification problem. Finding the optimal set of parameters to maximize accuracy is often an art in itself.

Finally, just feeding the training data into an algorithm and hoping for the best is typically a fast track to poor performance (if it works at all). Significant time is needed to clean the data, correct formats and add additional ‘features’ to maximize the predictive capability of the algorithm. We will go into more detail on both of these requirements in future installments.

OK, so now let’s put all this into context by looking at the competition I entered, provided by Airbnb. The aim of the competition was to predict the country that users will make their first booking in, based on some basic user profile data[2]. In this case, the categories were the different country options and an additional category for users that had not made a previous booking through Airbnb. The training data was a set of users for whom we were provided with the correct category (i.e. what country they made their first booking in). Using the training data, I was required to train the model to accurately predict the country of first booking, and then submit my predictions for a set of users for whom we did not know the outcome.

How?

The aim of this series is to walk through the process of assessing and analyzing data, cleaning, transforming and adding new features, constructing and testing a model, and finally creating final predictions. The primary technology I will be using as I walk through this is Python, in combination with Excel/Google Sheets to analyze some of the outputs. Why Python? There are several reasons:

  1. It is free and open source.
  2. It has a great range of libraries (also free) that provide access to a large number of machine learning algorithms and other useful tools. The libraries I will primarily use are numpy, pandas and sklearn.
  3. It is very popular, meaning when I get stuck on a problem, there is usually plenty of material and documentation to be found online for help.
  4. It is very fast (primarily the reason I have chosen Python over R).

For those that are interested in following this series but do not have a programming background, do not panic – although I will show code snippets as we go – being able to read the code is not vital to understanding what is happening.

Next Time

In the next piece, we will start looking at the data in more detail and discuss how we can clean and transform it, to help optimize the model performance.

 

[1] This is an actual competition on Kaggle at the moment (no prizes are awarded, it is for experience only).

[2] The data has been anonymized so that users cannot be identified

 

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