- The term "crossover" is mainly used to describe experiments in which all individuals pass through both the treatment and placebo phases. In the case crossover design, the treatment phase is the hazard period and placebo phase is the control period. Since there are a hazard period and a control period and each individual provides the exposure information for both hazard and control periods, case crossover design can be viewed as a matched case-control study design where it involves cases only and each individual serves as his/her own control. 

- On the other hand, the case crossover study can also be viewed as a retrospective cohort study as the control data are not necessarily the "counts", they can be units of exposure person-time (this will be illustrated in slides 13, 16, 20). When it is this case, the analysis for the study can be treated as a pooled analysis of several cohort studies each with a sample size of one.