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The decision on what studies to include in a meta-analysis should be based on a focused research question. Some of the things to consider are (1) types of study designs to include (for example, randomized trials versus randomized and nonrandomized trials), (2) types of subjects to include (for example, gender, age, medical condition(s), etc.), (3) types of publications (for example, published journal articles only versus published and unpublished articles), (4) language restrictions, if any (for example, English-language articles only versus all), (5) types of interventions (for example, aerobic exercise only versus aerobic and progressive resistance exercise), and (6) time frame for included studies (for example, studies conducted between January 1960 and December 2000).