![]() If it’s a natural process, we would have seen the same change even if the subjects did not participate in the experiment. If the posttest scores are higher at the end, we might be observing regular knowledge acquisition rather than the program causing the increase. Imagine if instead of a weight loss program, we are studying an educational program. The change between pretest and posttest scores might represent a process that occurs naturally over time and, thus, raises questions about internal validity. ![]() It might be the fitness program that caused the weight loss rather than the weight loss program we’re studying. Perhaps a fitness program became popular in town, and many subjects participated. ![]() HistoryĪn outside event occurred between the pretest and posttest that affected the outcomes and can reduce internal validity. The following items are threats to internal validity. However, there are various threats to attributing a causal connection between the weight loss program and the changes in weights.
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