Assess the impact of moderator variables

When the effect size varies substantially from study to study an important goal of the meta-analysis could be to understand the reason for this variation. 

Use analysis of variance to assess the impact of categorical moderators.  For example, “Is the treatment more effective for acute patients than for chronic patients?” or “Is homework a more effective intervention than tutoring?”

Use meta-regression to assess the impact of continuous moderator variables.  For example, “Does the treatment effect increase as a function of dosage?”, or “Is the magnitude of the effect size related to the age of the students?”

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