Some issues in the statistical analysis of completely randomized and repeated measures designs for speech, language, and hearing research

Contemporary investigators in the areas of speech, language, and hearing rely heavily on inferential statistical procedures to answer both basic and applied research questions. Such statistical procedures typically involve a number of assumptions that need to be fulfilled in order for the procedure to be appropriate for a specific data set. Unfortunately, a review of recent publications in the Journal of Speech, language, and Hearing Research indicated that some pivotal issues related to those underlying assumptions, although widely discussed and emphasized in the statistical literature, often appear to be neglected in these Fields of research. This tutorial therefore addresses two issues that are particularly important for an appropriate and accurate use of some of the most commonly used statistical procedures. The first issue concerns the importance of addressing the sphericity assumption in studies with a repeated measures design. The second issue concerns the definition of the experimental units in a statistical analysis and applies to both completely randomized and repeated measures designs. Theoretical aspects associated with each issue are discussed, and appropriate strategies for data entry and analysis are presented.