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Internal and external validity

I don't understand what internal and external threats are and could you provide some examples of what would be considered a threat to internal and/or external validity of a study design.

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In experiments, we manipulate an independent variable and measure its effect on the dependent variable. Internal validity addresses the issue of whether your independent variable was the cause of the change in the dependent variable. There are several threats to this. For example, imagine you have designed a study to see if the time of day influences how well people can remember a list of words. You measure some people in the morning and others at night. In order to maintain good internal validity, you must ensure that everything except the time of day is the same between the groups. If you test the two groups in different rooms, with different experimenters, on different days of the week, or with different lists of words, that could affect their performance and lead you to think (incorrectly) that the time of day is what drove their performance. We would call those factors extraneous variables - they are variables that were not adequately controlled and thus influenced your results.

A classic example of a problem with internal validity is the Hawthorne effect. This effect is named after a study done at the Hawthorne electrical company several decades ago. Researchers were interested in whether the brightness of lighting in the factory influenced the productivity of workers. They increased the lighting and found that productivity increased. This made ...

Solution Summary

Explains internal and external validity using examples in Psychology.