The failure of readability formulas can be attributed to three weaknesses in the formulas. First, they ignore or violate current knowledge about the reading process. Most formulas affect only sentence length and word difficulty while ignoring factors that influence text comprehensibility, such as cohesion, the number of inferences required, the number of items to remember, complexity of ideas, rhetorical structure, dialect, and required schemata. Nor do they account for reader-specific factors such as interest and the purpose for reading. Second, readability formulas lack solid statistical grounding. The most respected formulas have been validated by test lessons that were intended only as practice exercises, never as measures of text comprehensibility or as indicators of reading ability across age, class, or cultural groups. Third, readability formulas are used inappropriately in two of the contexts in which they appear to be most valuable. Even a formula with some validity, used with appropriate texts and readers, cannot correctly predict how a particular reader will interact with a particular book. (HTH)