There are lots of different readability formulas out there, which seek to provide an index on how readable any given excerpt of text is. Typically, these formulas output a grade-level score, which indicates, roughly, the level of education required to read the text excerpt with ease. Any “quality index” that seeks to reduce the complexity of something as multi-faceted as reading should be subject to scrutiny. This is true for Bounce Rate, this is true for Time On Page, and this is true for a readability score.
When looking at Google Analytics reports, you'd think you get a pretty good idea of how people are interacting with your site, right? I mean, you're tracking events here, pageviews there, and user timings, custom dimensions, custom metrics, and calculated metrics are all part of your daily lingo. But you're also probably aware of how futile this tracking is. After all, all you're seeing are numbers that reflect certain outcomes the visitors have produced on the website, and how these outcomes match against your preconceived goals and objectives, right?