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At Cmotions, we love a challenge. Especially those that make us both think and have fun. Every now and then we start a so-called ‘Project-Friday’ where we combine data science with a fun project. In the past we build a coffee machine with visibility, an algorithm for handsign recognition, a book recommender and a PowerBI escaperoom (see our projects). A few months ago, we decided to join a Kaggle competition: the CommonLit Readability, for numerous reasons. First of all, for many of us this is the first time  attending a Kaggle competition; high on many bucket lists. Not to mention we like to do more with textual data to get a deeper knowledge of NLP techniques. And last but not least, we like to help kids to read stuff that meet their profile. 

In the CommonLit competition this question needs to be answered: 

“To what extent can machine learning identify the appropriate reading level of a passage of text, and help inspire learning?” 

This question arose as CommonLit, Inc. and Georgia State University wanted to offer texts of the right level of challenge to 3rd to 12th grade students, in order to stimulate the natural development of their reading skills. Until now, the readability of (English) texts, has been based mainly on expert assessments, or on well-known formulas, such as the Flesch-Kincaid Grade Level, which often lack construct and theoretical validity. Other, often commercial solutions also failed to meet the proof and transparency requirements. 

To win a Kaggle competition…

Before we entered the competition, we did see some drawbacks. Often, a high rank in a Kaggle competition these days means combining many, many, many models to gain a bit of precision and a lot of rank positions – there’s not a lot of difference in accuracy in the highest ranked contenders. With the increase of predictive performance there’s a steep decrease in interpretability when combining the results of many models into a final prediction. Also, since this challenge is about textual data and we do not have many train cases, state of the art pretrained transformer models can be useful. Downside,these are complex and difficult to understand. Let alone when we combine multiple transformer models for our best attempt to win the competition…

What we did differently

No mistake about it, when we join a competition, we’re in it to win it. But we wanted to maximize learning and explainability of our approach. Therefore, where we expected many others to focus on finetuning and combining transformer models, we’ve spent much of our available time on thinking of relevant features we could create that help in understanding why our model predicts one text is more complex to read for kids than another text. We brainstormed, read the literature and checked with our friends and family with positions in education. This resulted in an impressive set of 110 features -derived from the text. These features both include readability scales from the literature – Flesh reading ease, Flesh Kinraid grade, Dale Chall Readability score – as well as other characteristics we thought might help in explaining the readability of text – sentence length, syllables per word, abbreviations, sentiment, subjectivity, scrabble points per word, etcetera

Since we think understanding the why is as important as most predictive power, we followed this approach:

Model to explain readability

We started listing potential features and started creating them in the limited time we had available for this. After this feature engineering, we estimated XGBoost models to identify the most important features and included those features in a regression model. This approach resulted in valuable insights:

  • About 55% of the variance in the readability score is explained [R2] by our features
  • The well-known Dale Chall readability score is by far the most predictive scale
  • Many extra features we derived help in predicting and explaining readability

Based on these results, we can think of ways how to help publishers, teachers and students how to write texts that are easier to read.

Model to boost predictions

Yet, we were not done with a model that is doing a good job in explaining. In the second stage, we explored if transformer models (BERT, Roberta, Distilbert) could help boosting our predictions. And it did! We knew these models, pretrained on huge amounts of textual data, are state of the art and excellent in many NLP tasks. Also in our case, RSME decreased from around 0.70 to 0.47 and the explained variance [R2] increased to 79%! After hyperparameter optimization a tuned Roberta Large with some advanced sampling was our best model.

And the winner is…

[spoiler alert!] No, we didn’t win this Kaggle competition. We ended somewhere in the upper half. To be honest, quite soon we knew that our approach would not be the one that would lift our bank accounts (the winner Mathis Lucka won $20K, congrats to him!). For this type of competition, the winner spends all time and resources on building many models and combining those, squeezing every last bit of predictive power out of the train data. For example, see the approach of the runner up. It should be noted however, that the winner, Mathis Lucka did use a bunch of models and ensembled those, but also had an original approach to involve smart new data (see his Kaggle post for details).

Even though we didn’t win, we enjoyed the competition very much and we’ve learned a lot. Also, we believe that our approach – have a model to interpret to explain the why and if needed add a booster to maximize predictions is a winning approach in many real life (business) contexts. (And we believe that an approach that ensembles 10+ transformer models is not ?). Therefore, we have already started translating what we learned in this Kaggle competition into something we can help others with to get and use more readable texts. Curious what exactly? Stay tuned and you’ll find out!

Principal Consultant & Data Scientist
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