Klare concluded that in the studies that showed increased comprehension, transforming text requires attending to other problems besides word and sentence length. "The best assumption, it seems to me," he wrote, "is that the research workers, probably with considerable effort, managed to change basic underlying causes of difficulty in producing readable versions" (p. 148). Klare then listed the following word-and-sentence variables that affected comprehension:

Word characteristics:

  1. Proportion of content (functional) words.
  2. Frequency, familiarity, and length of content words.
  3. Concreteness or abstractness.
  4. Association value.
  5. Active vs. nominalized verb constructions.

Sentence characteristics:

  1. Length (esp. clause length).
  2. Active vs. passive.
  3. Affirmative vs. negative.
  4. Embedded vs. non-embedded.
  5. Low depth vs. high depth (branches).

Since Klare's 1976 study, there have been other studies showing the positive effects of using formula variables to improve comprehension (Ewing 1976, Green 1979, C. C. Swanson 1979).

In the many studies of before-and-after revision of the text, a negative result does not prove that there is no improvement in comprehension. They show instead that improvement has not been detected. There is a saying in statistics that you cannot prove a negative.

Studies reporting a negative result may result from failing to control the reading ability, prior knowledge, interest, and motivation of the subjects. They can also result from failing to control elements of the text such as organization, coherence, and design. The great difficulty of properly conducting such an experiment is seen in following two studies.

The Duffy and Kabance study Critics worry that technical communicators can too easily misuse the formulas, making documents more difficult, not less (Charrow 1977, Kern, 1979, Selzer 1981, Lange 1982, Duffy 1985, Redish and Selzer 1985, Connaster 1999, Redish 2000, Schriver 2000). These writers offer little or no evidence of such misuse, however, widespread or otherwise. If unscrupulous or careless writers choose to cheat by "writing to the formula" and not attending to other textual issues, careful editors and reviewers easily spot the misuse. The study by Thomas Duffy and Paula Kabance (1981) is a case in point. Because formula critics (e.g., Redish and Selzer 1985; Redish 2000) often refer to this study, it deserves some attention.