There has been extensive research into developing ways to create valid tests. Hundreds of papers have been written and at least 200 different formulas have been designed over the last 50 or 60 years to find the best way to measure text readability.
Initially the focus was on measuring text difficulty for schoolchildren, but in 1934, Dale and Tyler published a readability formula for use with adults. Many more have been developed since then. Most of these are based on some formula of sentence length and word length. For example, Gunning’s Fog Index is based on the percentage of words with three or more syllables in a document. Other refinements sometimes include counting the number of words which are not in a standard collection of familiar words; or counting the number of adjectives and adverbs in the sentence.
The US military has done literacy testing over many decades in order to produce materials for recruits. It has had the luxury of large numbers of substantially similar participants to test, making it simpler to draw conclusions from the results.
Rudolf Flesch
Rudolf Flesch, in 1949, wrote an account of his Flesch Reading Ease scale
(FRE). This scale is still commonly used today. It measures from 100 to 0, with
higher numbers meaning easier reading. For example, Our New Friends (a
Dick and Jane book) measures at 100 FRE (meaning most people can understand
it); Martin Luther King’s famous “I Have a Dream” speech scores 62.5;
chapter one of Albert Einstein’s Relativity: The Special and General Theory is
39.7; and a couple of paragraphs picked at random from a standard set of
condominium by-laws measures at FRE 9 (meaning almost no one can understand
them).
Many mission statements measure at 0, which means they are really high-level reading. This, however, is not altogether fair, as FRE needs at least 200 words to give a fair measurement. It is interesting, though, that organizations are so prone to squeezing a lot of long words into one long, unmemorable sentence.
Flesch-Kincaid Reading Level
Rudolf Flesch also worked together with J.P. Kincaid, to create a further test.
Flesch-Kincaid gives measurements in the form of a school grade level.
Readability tools on your computer
You can usually find both FRE and Flesch-Kincaid on your computer, as part
of whichever word-processing software you use. For instance, in Microsoft
Word XP, it is in the spelling and grammar section. After checking your
grammar, you can view the readability statistics. These include a count of
the number of words, sentences and paragraphs. Below that are the average
number of sentences per paragraph, words per sentence, and characters per
word. And below that are readability figures—the percentage of passivevoice
verbs used, the Flesch reading level and the Flesch-Kincaid level.