Where:

easy words = number of number of 1 and 2-syllable words per 100 words

hard words = number of words of more than 2 syllables per 100 words

sentences = number of sentences per 100 words

Flesch Reading Ease formula simplified and converted to grade level (now known as the Flesch-Kincaid readability formula):

New:

GL = (.39 x ASL) + (11.8 x ASW) - 15.59

Simplified:

GL = ( .4 ASL ) + ( 12 ASW ) - 15

Where:

ASL = average sentence length (the number of words divided by the number of sentences).

ASW = average number of syllables per word (the total number syllables in the sample divided by the number of words).

The second part of the study looked at the relationship between readability and learning time. It monitored the progress of 200 Navy technical-training students through four modules of their course for both comprehension and learning time. The study was replicated with a secondary sample of 100 subjects performing on four additional modules.

The results of the comprehension test showed the highest percentage of errors in both the readers with the lowest reading grade levels and in the modules with the highest grade-levels of readability.

In the same manner, the learning time systematically decreased with reading ability and increased with the difficulty of the modules. The study confirms that learning time as well as reading ability are significant performance measures for predicting readability.

The new Flesch-Kincaid formula was able to predict significant differences between modules less than one grade level apart using both comprehension scores and learning times. The U.S. Department of Defense (1978) authorized this formula in new procedures for validating the readability of technical manuals for the Armed Services. The Internal Revenue Service, and the Social Services Administration also issued similar directives.

Both Kern (1979) and Duffy (1985) urge the military to abandon use of the formulas. They note that writers in the military often find the task of simplifying texts below the 10th grade "too difficult" and "not worth the trouble." Unfortunately, there are no practical alternatives to the skill hard work required to create simple language. When large numbers of readers are involved, even small increases in comprehension pay off.

The Hull formula for technical writing At the 1979 STC conference, Leon C. Hull (1979) argued that technical writing, with its increased use of difficult words, needs a special kind of formula. While acknowledging that the FORCAST and Kincaid formulas were developed precisely for that reason, he looked for a formula that does not use word length as a variable.

Basing his work on Bloomer (1959) and Bormuth (1969) as well as his own experience as a technical writer, Hull claims that an increase in the number of adjectives and adverbs before a noun lowers comprehension. His study indicates that the modifier load is almost as predictive as a syllable count, more causal, and more helpful for rewriting.