Donald Murphy (1947), the editor of Wallace's Farmer, used a split run with an article written at the 9^th-grade level on one run and on at the 6^th-grade level on the other run. He found that increasing readability increased readership up of the article 18 percent. In a second test, he took great care not to change anything except readability, keeping headlines, illustrations, subject matter and the position the same. He found readership increases of 45% for an article on nylon and 60% for an article on corn.

Wilbur Schramm (1947) showed that a readable style contributes to the readers' perseverance, also called depth or persistence, the tendency to keep reading the text.

Charles E. Swanson (1948) showed that better readability increases reading perseverance as much as 80 percent. He developed an easy version of a story with 131 syllables per 100 words and a hard version with 173 syllables and distributed each to 125 families. A survey of readers taken 30 hours after distribution showed a gain in the easier version over the hard version of 93% of total paragraphs read, 83% in mean number of paragraphs read, and 82% in the number of correspondents reading every paragraph.

Bernard Feld (1948) grouped 101 stories from the Birmingham News into those with high Flesch scores, requiring 9^th-grade education or more and those with low scores, requiring less than 9^th-grade education. He found readership differences of 20 to 75 percent favoring the low-score versions. Feld's findings indicated that even a small actual percentage gain for a large-circulation paper greatly increased the number of readers.

Reading efficiency Klare, Shuford, and Nichols (1957) followed up these studies with a study of the reading efficiency and retention of 120 male aviators in a mechanics course at Chanute Air Force Base in Illinois. They used two versions of technical training materials, hard (13^th-15^th grade) and easy (7^th-8^th grade).

They measured reading efficiency with an eye-movement camera with which they could determine the number of words read per second and the number of words read per fixation. A strong "set-to-learn" was stimulated by allowing the subjects to re-read the text and giving them a pre-test before the experimental test.

The study showed that the easy text significantly improved both reading efficiency and retention. The results also indicated that a strong "set to learn" improved scores.

Hardyck and Petrinovich (1970) showed the connection between readability and both comprehension and muscle activity in the oral area (subvocalization).

Rothkopf (1977) showed the connection between readability and how many words a typist continues to type after the copy page is covered (functional chaining).

Readability and course completion Publishers of correspondence courses are understandably concerned when large numbers of students do not complete the courses. They often suspect the materials are too difficult for the students. Working with Kim Smart of the U. S. Armed Forces Institute, Klare (1973) applied the Flesch Reading Ease formula to thirty sets of printed correspondence courses used by the military.

They found that two of the high school courses and five of the college courses were too difficult for readers of average or below average reading skill.

They then compared their reading analysis to the completion records of the 17 courses that had been in use over two years. They found a Spearman rank-order correlation of .87 between the readability score and the probability of students completing the course. There was a Pearson product-moment correlation of .76.