Why readability formulas fail

Being able to measure the readability of a text with a simple formula is an attractive prospect, and many groups have been using readability formulas in a variety of situations where estimates of text complexity are thought to be necessary. The most obvious and explicit use of readability formulas is by educational publishers designing basal and remedial reading texts; some states, in fact, will consider using a basal series only if it fits certain readability formula criteria. Increasingly, public documents such as insurance policies, tax forms, contracts, and jury instructions must meet criteria stated in terms of readability formulas.Unfortunately, readability formulas just don't fulfill their promise. This failure can be attributed to three weaknesses in the formulas. From a theoretical point of view, they ignore or violate much of current knowledge about reading and the reading process. Second, their statistical bases are shaky, being at once poorly supported mathematically and difficult to generalize. Finally, as practical tools either for matching children and texts or for providing guidelines for writers they are totally inappropriate. Criticisms such as these have been leveled at readability formulas from many quarters (Gilliland, 1972; Redish, 1979; Kintsch & Vipond, 1977), but the formulas' uses have expanded in spite of the growing number of papers discussing their weaknesses. We attempt here to categorize and summarize some of the problems with readability formulas and their use.