Threats to Internal Validity

  • Aside from unrecognized differences prior to the start of a study, other factors play can threaten internal validity
    1. Mortality
      • If study drop out rates or the type of subjects who drop out are different between groups, the result could significantly skew the results
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      • Hawthorne Effect
        • The mere fact that a person is being observed or is taking part in a study can affect his or her behavior. If the effect is different between groups, internal validity is again threatened