This is the rationale behind readability estimates. Almost all readability estimates use some measure of word difficulty and syntactic complexity as their main predictors. Examples of word difficulty predictors are word length, syllable counts, and the number of unfamiliar words. Examples of sentence difficulty predictors are the number of dependent clauses and average sentence length. Of these, the number of unfamiliar words and average sentence length have proven to be the best predictors.
Many factors contribute to the readability of a passage.
Size of print, illustrations, and inherent difficulty of the subject matter are but a few. While readability estimates can be lowered by breaking long sentences into shorter ones and by using more common synonyms of unfamiliar words, most studies show that very little, if any, increases in comprehension is gained by this practice. In addition, it is important to understand what the various readability estimate formulas will not do. They do not take into account many of the most important aspects of the material and its relationship to the reader. They do not take into account the interest areas and interest level of the material.