Surprisal Takes It All: Eye Tracking Based Cognitive Evaluation of Text Readability Measures

Text readability measures are widely used in many real-world scenarios and in NLP. These measures have primarily been developed by predicting reading comprehension outcomes, while largely neglecting what is perhaps the core aspect of a readable text: reading ease. In this work, we propose a new eye tracking based methodology for evaluating readability measures, which focuses on their ability to account for reading facilitation effects in text simplification, as well as for text reading ease more broadly. Using this approach, we find that existing readability formulas are moderate to poor predictors of reading ease. We further find that average per-word length, frequency, and especially surprisal tend to outperform existing readability formulas as measures of reading ease. We thus propose surprisal as a simple unsupervised alternative to existing measures.