Prior knowledge and retention A series of studies in the military (Klare et al. 1955a) examined how prior knowledge as well as the text variables affect the retention and the acceptability of technical documents.

The studies were conducted at Sampson Air Force Base in New York and Chanute Air Force Base in Illinois using 989 male Air Force enlistees in training with different versions of the same texts. They used the Flesch Reading Ease, Dale-Chall, and the Flesch Level-of-Abstraction formulas to rate the texts as Easy (grade 7), Present (12^th grade), and Hard (16^th grade).

While simplifying documents and changing the style, they retained all technical terms and used technical experts to assure that they did not change the content.

This study found the more readable versions resulted in:

• Greater and more complete retention.
• Greater amount read in a given time.
• Greater acceptability (preference).

The study found that, "...while style difficulty appears to affect immediate retention of subjects who are naïve regarding material, subjects who have considerable knowledge of the material may profit little if any from an easier style of material" (p. 294).

Duffy (1985) criticizes the results of this study. He states that the 8% percent improvement in comprehension, achieved by dropping the reading level of the texts eight grades (from the 16^th+ grade to the 7^th-8^th grades - 1% improvement for each grade dropped) is not large enough to justify the effort required.

Duffy underestimates the difficulty of demonstrating the comprehension gained by changing any textual variable while carefully controlling the other variables done in the study. Most researchers are very happy to get any non-chance improvements in comprehension, the holy grail of reading research.

The difficulty arises from the complexity of reading comprehension and the means we have of testing it, which are all indirect. Researchers, for example, are not sure exactly what the results of reading tests are telling them. Do they reveal comprehension of the text or other artifacts such as prior knowledge, memory, or the difficulty of the questions?

Studies of the effects of textual variables and writing strategies on comprehension are very often inconsistent, inconclusive, or non-existent. Examples include: the use of illustrations (Halbert 1944, Vernon 1946, Omaggio 1979; Felker et al., 1981), schemas (Rumelhart 1984), structural cues (Spyridakis, 1989, 1989a), highlighting (Klare et al. 1955b, Felker et al.), paragraph length (Markel, et al., 1992), typographic format (Klare 1957), syntax simplification (Ulijn and Strother 1990), prior knowledge (Richards 1984), nominalizations, diagrams, parallelism, white space, line graphs, and justified margins (Felker et al,), "whiz deletions" (Huckin et al. 1991), writer guidelines (McLean 1985), and coherence and cohesion (Freebody and Anderson 1983, Halliday and Hasan 1976).

No one would say that any of these items are not helpful or do not affect comprehension. These studies show, however, how difficult it is to detect and measure the effect on comprehension of any reading variable. Even small gain in comprehension that is significant can be important over time and suggests further study. In this regard, the formulas do very well. See "Producing and transforming text" below.