Readability Formulas in the New Millennium: What's the Use?
http://doi.acm.org/10.1145/344599.344638
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  peer-reviewed

Schriver, Karen A.
Journal of Computer Documentation
2000

Abstract:

While readability formulas were intended as a quick benchmark for indexing readabilty, they are inherently unreliable: they depend on criterion (calibration) passages too short to reflect cohesiveness, too varied to support between-formula comparisons, and too text-oriented to account for the effects of lists, enumerated sequences,and tables on text comprehension. But readability formulas did spark decades of research on what comprehension really involoves.