A selection effect is a type of confounding variable that occurs when the participants in a treatment group differ from those in a control group in some systematic, pre-existing way. This issue frequently arises in quasi-experimental designs where researchers cannot use random assignment to place participants into conditions. When a selection effect is present, it provides an alternative explanation for the results, making it difficult to confidently determine whether the observed differences in the dependent variable were caused by the experimental treatment or by the inherent differences between the groups.