11 - 9/28/11 Threats to Internal Validity History Events...

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9/28/11 Threats to Internal Validity History – Events outside the lab between pre and post-test Maturation – naturally occurring change during the experiment – hunger, fatigue, boredom Testing – effects of repeated tests – practice, familiarity, etc. Mortality – when subjects drop out of the study Selection – samples are not random, or, by chance, on sample differs from the other Regression to the mean – subjects with extreme scores are likely to score closer to the mean the second time Threats to External Validity Interaction between subjects and treatment: Population validity o Are Psyc 107 students representative? Interaction between setting and treatment: Ecological validity o Will we get the same result in India Interaction between history and treatment: Temporal validity o What if we held our experiment the day after 9/11? o End of the Semester? Procrastinating subjects
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11 - 9/28/11 Threats to Internal Validity History Events...

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