For example, they scored a 400 when all other times are usually around 300 and vise versa: regression- tends to happen when subjects are selected because they score high or low on some variable. When they"re re-tested, their scores tend to change in the direction of the mean. Very high scores become lower and very low scores become higher: testing & instrumentation (testing effect: practice and memorization may influence second test. Alterations to the test may affect internal validity by, for example, an unknown change in test difficulty) Subject-selection effect (threatens internal validity) this occurs when subjects can"t be randomly assigned. Subject attrition- refers to subject failing to complete a study. Those finishing the study may not be equivalent to those dropping out of the study. Between-subject design- a subject either receives level a or level b but. When you should use this design: subject variable, deception is involved, demand characteristics exist if practice effects are an issue.