Regarding our previous article on controlled clinical trials, this essay extends the discussion to the classification of sequential controlled trials.
Sequential controlled clinical trials are conducted in two forms of self-control and cross-over. In the self-control design, patients in the control group are those participated in the experimental group. Since the demographic variables of the participants in the experimental and control groups should be similar; therefore, participants in the control group are the best match for the experimental group. However, it should be mentioned that different timing of measurement and period effects may affect the outcomes of the treatment in this type of research.
Cross-over studies are derived from self-control and parallel controlled designs. In this regard, two groups of participants are employed; one as the experimental and the other one as the control group. The experimental group receives the intervention, whereas the control group receives a placebo. In the next step, participants in both groups receive no treatment for a period of time, which is referred to as a wash out period. The main aim of the wash out period is to eliminate the effect of the treatment. Following that period, participants in the control group play the role of cases in experimental groups and receive the treatment, and those the experimental group are considered as the control group. One of the main advantages of cross-over design is that a small sample size would suffice the initiation of the study since each group is subjected to both placebo and treatment.
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