Advancing Group-based Disparities Research and Beyond: A Cautionary Note on Selection Bias

Obtaining an accurate understanding of group-based disparities is an important pursuit. However, unsound study designs can lead to erroneous conclusions that impede this crucial work. In this paper, we highlight a critical methodological challenge to drawing valid causal inferences in disparities research – selection bias. We describe two commonly adopted study designs in the literature on group-based disparities. The first is outcome-dependent selection, when the outcome determines whether an observation is selected. The second is outcome-associated selection, when the outcome is associated with whether an observation is selected. We explain the methodological challenge each study design presents and why it can lead to selection biases when evaluating the actual disparity of interest. We urge researchers to recognize the complications that beset these study designs and to avoid the insidious impact of inappropriate selection. We offer practical suggestions on how researchers can improve the rigor and demonstrate the defensibility of their conclusions when investigating group-based disparities. Finally, we highlight the broad implications of selection mechanisms for psychological science.