Readability formulas have been around for many years, and one
of the earliest developed, the Flesch Reading Ease formula, is still
among the most commonly used. Rudolf Flesch (1948) established
a model for formula development: identify the key variables that
you think affect dif!culty; produce versions of a text that alter those
variables; test those versions on people who can be strati!ed in
some easily communicated way (ie, school grade levels); then relate
the count of those variables to the ability levels of the readers who
succeeded in reading successfully.