RM internal validity

Selection Bias

When characteristics of the experimental and comparison group subjects differ in any way that can influence the outcome. Differential attrition: occurs when comparison groups
become different because subjects are more likely to drop
out of one of the groups than the other for various reasons

Endogenous Change

When the subjects develop or change during the experiment as part of an ongoing process independent of the experimental treatment. Testing: pretest can influence posttest scores. Maturation: age, gain experience and knowledge regression effect: Subjects chose because of extreme score become less extreme due to episodic change.

External Events

When something occurs during the experiment, other than the treatment, that influences outcome scores

Contamination

When either the experimental group or the comparison group is aware of the other group and is influenced in the posttest as a result. Compensatory rivalry: Comparison group becomes aware that they are denied some advantage, increase effort to compensate. Demoralization: feel they have been left out of some valuable treatment and perform worse than they would have

Treatment Misidentification

When variation in the independent variable (the treatment) is associated with variation in the observed outcome, but the change occurs through an intervening process that the researcher has not identified. Placebo effect, Hawthorne effect: know they are a part of a study.