How often does random assignment fail? Estimates and recommendations

A fundamental goal of the scientific process is to make causal inferences. Random assignment to experimental conditions has been taken to be a gold-standard technique for establishing causality. Despite this, it is unclear how often random assignment fails to eliminate non-trivial differences between experimental conditions. Further, it is unknown to what extent larger sample sizes mitigates this issue. Chance differences between experimental conditions may be especially important when investigating topics that are highly sample-dependent, such as climate change and other politicized issues. Three studies examine simulated data (Study 1), three real datasets from original environmental psychology experiments (Study 2), and one nationally-representative dataset (Study 3) and find that differences between conditions that remain after random assignment are surprisingly common for sample sizes typical of social psychological scientific experiments. Methods and practices for identifying and mitigating such differences are discussed, and point to implications that are especially relevant to experiments in social and environmental psychology.