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Clinical Trials

3. Classification

Trial design

Trials can be further classified by design. This classification is more descriptive in terms of how patients are randomized to treatment.

Parallel-Group trials are the most common design (Pocock, 1983; Friedman, 1998). Patients are randomized to the new treatment or the standard treatment and followed-up to determine the effect of each treatment in parallel groups.

Crossover trials randomize patients to different sequences of treatments, but all patients eventually get all treatments in varying order, i.e., the patient is his/her own control (Senn, 2002; Jones & Kenward, 2003; Wang et al., 2006g).

Factorial trials assign patients to more than one treatment-comparison group that are randomized in one trial at the same time; i.e., while drug A is being tested against placebo, patients are re-randomized to drug B or placebo, making four possible treatment combinations in total (Fox et al., 2006).

Cluster randomized trials are performed when larger groups (e.g., patients of a single practitioner or hospital) are randomized instead of individual patients (Mallick et al., 2006b).

Cluster trials can be any of the previously mentioned designs.

Figure 2

Basic Trial Designs

A graphic depiction of basic trial designs including parallel,crossover and factorial as mentioned in the text. Note: Cluster randomized trials can be any of the previously mentioned designs.
Pocock SJ. (1983) Clinical trials: A practical approach. Chichester: John Wiley & Sons.
Friedman LM, Furberg CD, Demets D. (1998) Fundamentals of clinical trials, 3rd edition. New York: Springer Verlag.
Senn S. (2002). Cross-over trials in clinical research, 2nd edition. Chichester: John Wiley & Sons.
Jones B, Kenward MG. (2003). Design and analysis of cross-over trials, 2nd edition. London: Chapman and Hall/CRC.
Wang D, Lorch U, Bakhai A. (2006g) Crossover Trials. In: D Wang & A Bakhai, (Ed.s). Clinical trials: A practical guide to design, analysis and reporting. London: Remedica. 91-99.
Fox Z, et al. (2006) Factorial design. In: D Wang & A Bakhai, (Ed.s). Clinical Trials: A practical guide to design, analysis and reporting. London: Remedica. 101-112.
Mallick U, et al. (2006b) Cluster randomized trials. In: D Wang & A Bakhai, (Ed.s). Clinical Trials: A practical guide to design, analysis and reporting. London: Remedica.141-151.