It was not possible to ensure that the participant had no exposure to the selected books prior to the implementation of the research. Even if this had been checked prior to commencement of the baseline phase, given that (a) time was needed to find appropriate books and to write the range of lower level and higher level questions and (b) the study was conducted over a 4-week period, it could not be guaranteed that the child would not have exposure to one or more of the books subsequent to a prebaseline check. The fact that a small to negligible effect size was found for prior exposure to books indicates that this variable does not explain the differences in complexity of language found between the baseline and intervention phases