n. the loss of study participants over time. Attrition may occur for a variety of reasons (e.g., the nature of the data being collected, participant relocation, aversive or costly data collection procedures) and can threaten the external validity and internal validity of research. It also creates the potential for bias—individuals who drop out may have unique characteristics that are relevant to the phenomenon of interest such that the remaining sample is no longer representative of the population—and may reduce the power of statistical analyses.