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16.5.1  Introduction

It is not uncommon for clinical trials to randomize participants to one of several intervention groups. A review of randomized trials published in December 2000 found that a quarter had more than two intervention groups (Chan 2005). For example, there may be two or more experimental intervention groups with a common control group, or two control intervention groups such as a placebo group and a standard treatment group. We refer to these studies as ‘multi-arm’ studies. A special case is a factorial trial, which addresses two or more simultaneous intervention comparisons using four or more intervention groups (see Section 16.5.6).


Although a systematic review may include several intervention comparisons (and hence several meta-analyses), almost all meta-analyses address pair-wise comparisons. There are three separate issues to consider when faced with a study with more than two intervention groups.

  1. Determine which intervention groups are relevant to the systematic review.

  2. Determine which intervention groups are relevant to a particular meta-analysis.

  3. Determine how the study will be included in the meta-analysis if more than two groups are relevant.