Dosage explains individual differences in the outcomes of a prevention program for literacy problems

We investigated whether dosage was related to the outcomes of the computer-based early-literacy intervention Build! as implemented by schools. Two aspects of dosage were distinguished: duration (time spent on the intervention) and exposure (the number of lessons completed). Exposure was expected to mediate the relation between duration and intervention outcomes. Three aspects of duration were distinguished: session frequency, session length, and intervention weeks. Participants were 226 kindergartners from 45 schools. Letter knowledge and phonological awareness were assessed before and after the intervention. Findings showed that, controlling for performance at pretest, the relation between duration and preliteracy at posttest was completely mediated by exposure. Session frequency and intervention weeks showed the strongest relation with exposure and literacy outcomes. Session length had smaller effects. Overall, the findings underline the importance of dosage for the outcomes of a literacy intervention in a naturalistic setting, when the intervention is implemented by schools.