Gray and Leary's work stimulated an enormous effort to find the perfect formula, using different combinations of the style variables. In 1954, Klare and Buck listed 25 formulas for children and another 14 for adult readers. By 1981, Klare noted there were over 200 published formulas.

Research eventually established that the two variables commonly used in readability formulas-a semantic (meaning) measure such as difficulty of vocabulary and a syntactic (sentence structure) measure such as average sentence length-are the best predictors of textual difficulty.

Some experts consider the number of morphemes for each 100 words to be a major contributor to semantic (meaning) difficulty and the number of Yngve word depths (branches) in each sentence to be a major contributor to syntactic (sentence) difficulty. One study (Coleman 1971) showed that Flesch's index of syllables for each 100 words correlates .95 with morpheme counts. Another study (Bormuth 1966) found that the number of words in each sentence correlates .86 with counts of Yngve word depths. Measuring the average number of syllables per word and the number of words in each sentence is a much easier method and almost as accurate as measuring morphemes and word depths.

Formula limitations Readability researchers have long taken pains to recommend that, because of their limitations, formulas are best used in conjunction with other methods of grading and writing texts. Ojemann (1934) warned that the formulas are not to be applied mechanically, a caution expressed throughout readability literature. Other investigators concerned with the difficulty and density of concepts were Morriss and Holversen (1938) and Dolch (1939). E. Horn (1937) warned against the mechanical use of the word lists in the re-writing of books for social studies.

George Klare and colleagues (1969) stated, "For these reasons, formula scores are better thought of as rough guides than as highly accurate values. Used as rough guides, however, scores derived from readability formulas provide quick, easy help in the analysis and placement of educational material."

Readability researchers such as Flesch (1949, 1964, 1979), Klare and Buck (1954), Klare (1980), Gunning (1952), Dale (1967), Gilliland (1972), and Fry (1988) wrote extensively on the other rhetorical factors that require attention such as organization, content, coherence, and design. Using the formulas creatively along with techniques of good writing results in greater comprehension by an audience of a specified reading ability (Klare 1976, Chall and Conard 1991).

Irving Lorge: Consolidating the research. Irving Lorge (1938) published The Semantic Count of the 570 Commonest English Words, a frequency count of the meaning of words rather than the words themselves. He later he was co-author of E. L. Thorndike's last book, The Teacher's Word Book of 30,000 Words (1944).

Lorge was interested in psychological studies of language and human learning. At Columbia, he came under the influence of Bryson at the Readability Lab. He wanted a simple formula for predicting the difficulty of children's books in terms of grade scores. For the criterion for his own formula, Lorge (1939) used the 376 selections taken from the McCall-Crabbs Standard Test Lessons in Reading (1926). He then standardized those passages on the basis of questions answered by children in terms of the Thorndike-McCall Reading Scale. Using correlation techniques, he was able to show that various combinations of factors gave predictions of higher accuracy than the Gray-Leary formula.