Parallel trials

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In essence, an RCT is a study in which people are allocated at random to receive one of two (or more) clinical interventions.  Usually, one of the interventions is regarded as a standard of comparison or control, and the group of participants who receive it is called the control group.  The control can be conventional practice, a placebo, or no intervention at all.  The other groups are called the ‘experimental’, ‘intervention’ or the ‘treatment’ groups.  RCTs seek to measure and compare different events that are present or absent after the participants receive the interventions and these events are called outcomes.

Most RCTs have a parallel design.  In these studies (also called parallel trials or RCTs with parallel group design), each group of participants is exposed to only one study intervention.  For instance, if a group of investigators uses a parallel design to evaluate the effects of a new analgesic compared with those of a placebo in patients with migraine, they would give the new analgesics to one group of patients and placebo to a different group of patients.

Most of the information in the Trial Protocol Tool relates specifically to parallel group trials.

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This page was last updated 1st April 2005.