Many indexes or reading difficulty have arisen to help the writer simplify understanding. Lucid writing is less likely to contain long words and complex sentences. Several factors account for this. Abstractions are more likely to be expressed in long words than are concrete concepts and abstractions are harder to understand. The most familiar words tend to be shorter.
Readers are likely to loose their train of thought in complex sentences.
It is not surprising that simple indexes based on word length and sentence length are related to reading difficulty. But the indexes tell only part of the story. Interesting material, vivid language, familiar references, personal references, quotations, white space, effective organization, and credibility all contribute to reading ease. In addition, a reading index can be tricked by following the formula without attending to the other factors just mentioned. The meaning of a difficulty measure's index value or grade level is questionable.