Low-literacy readers have problems with vocabulary and syntax. Unfamiliar words and long sentences make text difficult for them. Most of the materials prepared for the average readers have to be rewritten for those with very limited reading ability.
For average readers, keep sentences short. But for low-literacy readers, keep sentences shorter. I recommend a low-literacy sentence length of less than 15 words. Of course, this is arbitrary. You may write a longer sentence if anything shorter may compromise the sense. But let the long sentence be the exception, rather than the rule. With a little bit of practice, you’ll find that a short sentence under 15 words can best communicate to readers with low literacy.
Every word in a sentence affects the readability of a low-literacy text. Interestingly, in such texts, polysyllabic words are not a factor of reading difficulty. In Adult Literacy (Basic Skills And Libraries), Gerry Bramley writes: “Adults with literacy problems often have an extensive ‘social sight’ vocabulary which will naturally include polysyllabic words. For example, an adult with severe reading problems might still be able to walk around a supermarket and recognize such signs, labels and directions as ‘vegetables’, ‘tomatoes’, ‘sausages’ and ‘checkout’. It is monosyllabic words, particularly conjunctions, adjectives and pronouns, which cause difficulties for adult literacy students.” (Page 5, Library Association Publishing, London, 1991)
So words, be they short or long, must be familiar. Writers of low-literacy texts may rely on a useful list of commonest words, found in the third edition of Martin Cutts’ Oxford Guide To Plain English (2009). This book also has a chapter on low-literacy plain English, based ‘on the practical knowledge of Janet Pringle, a Canadian expert on the needs of low-literacy readers’.
The rule of thumb once again: Let the low-literacy sentence length be under 15 words.