2. History. History includes events in addition to the independent variables under study, that occur between one measurement and another (e.g., between a pre-test and post-test). For example, in testing the effects of an exercise program on psychological well-being following heart attack, some participants joined a church, or received additional social support, or changed jobs. These extraneous (history) variables may account for some of the differences between pre- and post-test scores.
To weaken history as a rival hypothesis, researchers should use equivalent experimental and control groups (created by random allocation or matching). Since the groups are, logically, likely to have the same historical variables happening between pre- and post-test, differences in the outcomes aren’t likely to be the result of history.