Every statistical procedure carries with it certain assumptions that must be at least approximately true before the procedure can produce reliable, accurate results. Researchers often apply a statistical procedure to their data without checking on the validity of the assumptions of the procedure. If one or more of the assumptions of a given statistical procedure are violated, then misleading results will be produced by the procedure. It is important that those who analyze data be fully aware of the details of the statistical procedure they are using, including its companion assumptions. If one or more assumptions are violated, an alternative procedure must be used to obtain valid results.