What is a parallel groups design?

A research design where you apply the treatment and the control (or the two treatments) simultaneously to two separate groups of subjects. Contrast this with a crossover design where each subject gets the treatment and then the control (or the control and then the treatment) in sequence. Here is an example:

• In Harris et al (1999), 990 patients newly admitted to a coronary care unit were randomly assigned to either a control group or a treatment group that received remote intercessory prayer. Both groups received the normal standard of medical care. The patients did not know which group they were assigned to, and the outcomes were measured in a blinded chart review.

This webpage was written by Steve Simon on 2002-10-15, edited by Steve Simon, and was last modified on 2008-07-08. This page needs minor revisions. Category: Definitions, Category: Research designs.