In ‘The Average Sentence Length’, I suggested that a sentence should not be measured only in words but also in syllables and letters. And I gave this rule of thumb: “Over the whole document, make the average sentence length 15-20 words, 25-33 syllables and 75-100 characters.”
Look at this sentence from M.J. Moroney’s Facts From Figures: “Most people are little removed from average intelligence, but geniuses and morons tend to occur in splendid isolation.” Words = 18; Syllables (S) = 34; Letters (L) = 99. Excepting a minor syllabic transgression, Moroney’s sentence seems to flatter my rule of thumb.
These variables W, S and L are good predictors of the readability of a text. Independently and in combination, these factors constitute seven indices of readability — three are mono-variable, three di-variable and one tri-variable. Each index shows the years of schooling (1 to 17+) required to understand a particular text.
W-Index = W/2 = 18/2 = 9
S-Index = S/3 = 34/3 = 11.3
L-Index = L/10 = 99/10 = 9.9
WS-Index = (W/4) + (S/6) = (18/4) + (34/6) = 10.2
WL-Index = (W/4) + (L/20) = (18/4) + (99/20) = 9.5
SL-Index = (S/6) + (L/20) = (34/6) + (99/20) = 10.6
WSL-Index = (W/6) + (S/9) + (L/30) = (18/6) + (34/9) + (99/30) = 10.1
Writers and teachers may choose any one of the seven indices and use it to measure the readability of any text. They may try out all the seven on different texts and heuristically choose that index which may be the most reliable.