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An important yet understudied area of researcher discretion is the use of covariates in statistical models. Researchers choose which covariates to include in statistical models and their choices affect the size and statistical significance of their estimates. How often does the statistical significance of published findings depend on nontransparent and potentially discretionary choices? We use newly available replication data to answer this question, focusing primarily on observational studies. In about 40% of studies, we find, statistical significance depended on covariate adjustments. The covariate adjustments lowered p-values to statistically significant levels, not primarily by increasing estimate precision, but by increasing the absolute value of researchers' key effect estimates. In almost all cases, articles failed to reveal this fact. Our findings do not demonstrate that researchers exploit this discretion, but that, because of a lack of transparency requirements, the opportunities are rife.
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