Regression to the mean is most likely to be a threat to internal validity when

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Regression to the mean is a statistical phenomenon that occurs when a variable that is extreme on its first measurement tends to be closer to the average on its second measurement. This is most likely to threaten internal validity when a sample is specifically chosen because it had unusually low or high scores on the dependent variable, as any subsequent measurement is likely to show a change towards the mean that could be mistakenly attributed to the effect of the experimental manipulation rather than this statistical tendency.

Regression to the Mean Is Most Likely to Be a Threat