Assessing Bias from the (Mis)Use of Covariates: A Meta-Analysis Political ScienceSSMART

Gabriel Lenz and Alexander Sahn

An important and understudied area of hidden researcher discretion is the use of covariates. Researchers choose which covariates to include in statistical models and these choices affect the size and statistical significance of estimates reported in studies. How often does the statistical significance of published findings depend on these discretionary choices? The main hurdle to studying this problem is that researchers never know the true model and can always make a case that their choices are most plausible, closest to the true data generating process, or most likely to rule out alternative explanations. We attempt to surmount this hurdle through a meta-analysis of articles published in the American Journal of Political Science (AJPS). In almost 40% of observational studies, we find that researchers achieve conventional levels of statistical significance through covariate adjustments. Although that discretion may be justified, researchers almost never disclose or justify it.

The preprint for this project can be found here.