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A literature review reveals many technical weaknesses of readability formulas (when compared to direct usability testing with typical readers): they were developed for children s school books, not adult technical documentation;they ignore between-reader differences and the effects of content, layout, and retrieval aids on text usefulness; they emphasize countable features at the expense of more subtle contributors to text comprehension.
Janice C. Redish (Tue,) studied this question.