With the rise of the plain-language movement in the 1960s, several critics of the formulas claimed that the formulas do not test comprehensibility (Kern 1979, Duffy and Kabance 1981, Duffy 1985). The history of the formulas, however, shows that from the beginning their scores correlate well with comprehension difficulty as measured by reading tests. The formulas rate very well when compared with other widely used psychometric measurements such as reading tests (Chall and Dale 1995). Their validity correlations make them useful for predicting the comprehension difficulty of texts (Bormuth 1966).

Waples and Tyler: What adults read During the Depression in the '30s, adult education and the increased use of libraries stimulated studies in reading. Sociologists studied "who reads what and why over consecutive periods," looking at reading as an aspect of mass communication.

Douglas Waples and Ralph W. Tyler (1931) published What People Want to Read About, a comprehensive, two-year study of adult reading interests. Instead of using the traditional library circulation records to determine reading patterns, they interviewed people divided by sex and occupation into 107 different groups. It showed the types and styles of materials that people not only read but also want to read. It also studied what they did not read and why.

They found that the reading of many people is limited because of the lack of suitable material. Readers often like to expand their knowledge, but the reading materials in which they are interested are too difficult.

Ralph Ojemann: The difficulty of adult materials The year 1934 marked the beginning of more rigorous standards for the formulas. Ralph Ojemann (1934) did not invent a formula, but he did invent a method of assessing the difficulty of materials for adult parent-education materials. His criterion was 16 passages of about 500 words taken from magazines. He was the first to use adults to establish the difficulty of his criterion. He assigned each passage the grade level of adult readers who were able to answer at least one-half of the multiple-choice questions about the passage.

Ojemann was then able to correlate six factors of vocabulary difficulty and eight factors of composition and sentence structure with the difficulty of the criterion passages. He found that the best vocabulary factor was the difficulty of words as stated in the Thorndike word list.

Even more important was the emphasis that Ojemann put on the qualitative factors such as abstractness. He recommended using his 16 passages for comparing and judging the difficulty of other texts, a method that is now known as scaling (See "Text leveling" below). Although he was not able to express the qualitative variables in numeric terms, he succeeded in proving they could not be ignored.

Dale and Tyler: Adults of limited reading ability After working with Waples, Ralph Tyler became interested in adults of limited reading ability. He joined with Edgar Dale to publish (1934) their own readability formula and the first study on adult readability formulas. The specific contribution of this study was the use of materials specifically designed for adults of limited reading ability.

Their criterion for developing the formula was 74 selections on personal health taken from magazines, newspapers, textbooks, and adaptations from children's health textbooks. They determined the difficulty of the passages with multiple-choice questions based on the texts given to adults of limited reading ability.