Behavior Within a Clinical Trial and Implications for Mammography Guidelines

Within the Canadian National Breast Screening Study, and inuential clinical trial on mammography, a substantial fraction of women in the control group received mammograms, and some women in the intervention group did not. Using this behavior, random assignment, and a standard model, I nd two important relationships. First, I find heterogeneous selection: women more likely to receive mammograms are healthier. Second, I find treatment eect heterogeneity: women more likely to receive mammograms are more likely to experience harm from them. These findings imply that current guidelines unintentionally encourage mammography for healthier women more likely to experience harm from them.