Desirability of Repeatability
Test-retest reliability is desirable in measures of constructs that are not expected to change over time. For example, if you use a certain method to measure an adult's height, and then do the same again two years later, you would expect a very high correlation; if the results differed by a great deal, you would suspect that the measure was inaccurate. The same is true for personality traits such as extraversion, which are believed to change only very slowly. In contrast, if you were trying to measure mood, you would expect only moderate test-retest reliability, since people's moods are expected to change from day to day. Very high test-retest reliability would be bad, since it would suggest that you were not picking up on these changes.
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