What type of confounding variable occurs when participants in a treatment group differ from those in a control group in a systematic, pre-existing way?

In a study comparing the effectiveness of two different study methods, if a researcher allows students to choose which method they want to use rather than using random assignment, the study is susceptible to a selection effect.

A selection effect is a threat to internal validity where the participants in one group differ systematically from those in another group before the study starts. Match each research scenario with the specific pre-existing participant characteristic that is responsible for the selection effect.

A researcher offers a voluntary stress-management workshop to employees and compares their stress levels to those of employees who did not sign up. Arrange the following events in the correct logical sequence to illustrate how a selection effect develops and creates a threat to internal validity.

In psychological research, a 'selection effect' is most likely to occur when which of the following happens?

Selection effects occur when groups in a study are fundamentally different before an experiment even begins. Match each core concept with the statement that best explains its relationship to this research challenge.

A researcher concludes that a new exercise program significantly reduces anxiety after comparing a group of fitness-club members who volunteered for the program to a group of non-members who did not. To critically evaluate the internal validity of this study, one must identify that the _____ acts as a confounding variable, as the groups likely differed in their baseline physical health or motivation before the program ever started.

A researcher investigates the effect of a new math tutoring program by comparing the final exam scores of students who voluntarily attended the weekly tutoring sessions to the scores of students who chose not to attend. In this scenario, the study design is susceptible to a selection effect because the students in the treatment group may have had higher pre-existing motivation or academic ability than those in the control group.

A researcher compares the test performance of students who volunteered for a test-prep course to those who did not. Because the students were not randomly assigned, pre-existing differences in motivation between the groups represent a selection effect, which is a type of _____ variable that provides an alternative explanation for the differences in test scores.

A researcher compares a voluntary workplace exercise program to no program and claims the program reduces absenteeism. Order the steps of evaluating this claim to demonstrate how a selection effect invalidates the causal conclusion, starting from identifying the design limitation to the final evaluation of the causal claim.