However, relying on readability formulae to measure text accessibility is an approach that has many drawbacks (Benjamin, 2012; Dubay, 2004;Siddharthan, 2004), mainly re-lated to the fact that readability formulae only employ surface text features such as word or sentence length.
for example, the Flesch formula (Flesch, 1948) presented below is very widely-used but cannot take into account the difficulty autistic readers have in using abstraction (Minshew et al., 2002) or the difficulty aphasic readers have with passive voice or other syntactic constructions (Berndt et al., 1996).