Readability: Why are my scores falling out of the scorecard range?

Readability scores can be higher or lower than the scorecard provided by the test writers. Their scorecards typically cover the grade ranges within the US school system, but this does not mean that a score cannot be calculated that results in a number outside the score range. This article is intended to inform you about why this can sometimes happen.

How Scoring is Calculated

Most of the readability algorithms can produce results outside the score range, when they do not limit content size. It rarely happens for the algorithms that have a restriction in regards to content in order to calculate a score (Coleman-Liau, SMOG, Fog Index), but the other readability tests do not.

For example:
With Flesch-Kincaid if the content on the page is very small the readability score can be negative.
If there are 6 words, 15 syllables and 1 sentence the algorithm looks like this:
206.835 - 1.015(6/1) - 84.6(15/6) = -10.76

With Automated Readability Index (ARI) the variables are long words and sentences,

Characters = 200
Words = 40
Sentences = 1
4.71 (200/40) + 0.5 (40/1) - 21.43 = -1.43

In the example above, we can see that the page contains 40 words made of 200 characters in one sentence, resulting in the score of -1.43. Note that the sentence is considered very long by how Siteimprove defines a long sentence, and the content on the page is very short. 

Characters = 240
Words = 40
Sentences = 1
4.71 (240/40) + 0.5 (40/1) - 21.43 = 26.83

Note: How the content is formatted on a page (e.g. lists or bullet points) can greatly affect what is considered as content. Therefore, the readability function uses "best-effort" to determine a readability score and should only be used as an indication of the readability of the page. There are many variables that can affect the readability score (e.g. how the website is set up). The rule of thumb is to evaluate the page individually. Thereby you can decide to ignore the page for readability checks or edit the content on the page to produce a new readability score.