Meta-analysis
Dr. Brind et al. (1996) conducted a meta-analysis of 23 independent epidemiologic studies. It calculated that there was on average a relative risk of 1.3 (1.2 - 1.4) increased risk of breast cancer. The meta-analysis was criticized for selection bias by using studies with widely varying results, using different types of studies and not working with the raw data from several studies, and including studies that have methodological weaknesses.
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