Though there are several factors that make text difficult for the reader, classic readability relies on two effective variables (vocabulary and sentence length) to grade texts on a scale of 1 to 17+ years of schooling. Jeanne S. Chall and Edgar Dale, in their new Dale-Chall readability formula (Readability Revisited, 1995), present a revised list of 3000 words familiar to 80 % of 4th graders in the U.S. and tables for obtaining cloze scores and reading levels.
Edgar Dale’s original word list had only 769 words known to beginning readers. It was derived from Edward L. Thorndike’s Teacher’s Word Book and Madeline Horn’s list for the International Kindergarten Union. Dale expanded the list to 3000 words for the Dale-Chall readability formula of 1948.
The new Dale-Chall formula of 1995 is better than the original in that there are no complex arithmetic tasks. One counts the number of complete sentences in exact 100-word samples as well as the number of words not in the list. There are clear instructions for identifying familiar and unfamiliar words. The authors say: “We suggest that the analyst make a first approximation as to whether a word is familiar (‘elemental’) or unfamiliar (‘educated’). The list (and guidelines) should then be consulted to confirm the analyst’s judgment. With practice this procedure becomes quite rapid.” Once the number of complete sentences and the number of unfamiliar words are determined, the reading levels may be obtained from the tables.
The average reader has about eight years of schooling. I have discussed this in an article titled ‘The Average Reading Grade’. So if writers are writing for the average readers, what guidelines can be drawn from the new Dale-Chall formula?
Martin Cutts recommends an average sentence length of about 15-20 words. This means that in an exact 100-word sample, there must be at least five complete sentences. I also looked at those parts of the Dale-Chall tables which indicated 8-9 years of schooling. It is my considered opinion that if the number of unfamiliar words is more than 20%, then the text is not appropriate for the average reader.
The revised Dale list of 3000 words is not available online. You need to get a copy of Readability Revisited published by Brookline Books.
Here follow my two guidelines for writers writing for the average reader: 1. In exact 100-word samples, let there be no fewer than five complete sentences; and 2. Let the percentage of unfamiliar (‘educated’) words be no more than 20.