A parallel group study is a simple and commonly used clinical design which compares two treatments. Usually a test therapy is compared with a standard therapy.  The allocation of subjects to groups is usually achieved by randomisation. The groups are typically named the treatment group and the control group. Parallel group designs do not require the same number of subjects in each group, although often similar numbers are observed. The design is commonly used in randomised controlled trials. Statistical analysis often boils down to a simple t-test of the between group difference in the outcome, which is usually a mean or a proportion.

Example

• Thomas K.B. General practice consultations: is there any point in being positive? Br Med J (Clin Res Ed) 1987; 294:1200-1202