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How standard is a standard deviation? A cautionary note on using SDs to compare across impact evaluations in education

Guest post by Abhijeet Singh
Last week on this blog, David wondered whether we should give up on using SDs for comparing effect sizes across impact evaluations. I wish that question was asked more often in the field of impact evaluations in education, where such comparisons are most rife. In this post, I explore some of the reasons why such comparisons might be flawed and what we might do to move towards less fragile metrics.