sampling bias
a systematic and directional error involved in the choice of units, cases, or participants from a larger group for study. Sampling bias can threaten the internal validity of a study if there is a possibility that preexisting differences arising from the sampling process may interact with the variable of interest. Similarly, if the procedure used to choose participants tends to favor specially motivated individuals or people from a certain segment of society, there would be a threat to the study’s external validity (i.e., inferences to a larger population would not be viable). Sampling bias is associated with a lack of random sampling and with nonrandom assignment to conditions. Also called recruitment bias; selection bias. See also self-selection bias.