Test-retest reliability is the most common measure of reliability. In order to measure the test-retest reliability, we have to give the same test to the same test respondents on two separate occasions. We can refer to the first time the test is given as T1 and the second time that the test is given as T2. The scores on the two occasions are then correlated. This correlation is known as the test-retest-reliability coefficient or the coefficient of stability.
The closer each respondent’s scores are on T1 and T2, the more reliable the test measure (and the higher the coefficient of stability will be). A coefficient of stability of 1 indicates that each respondent’s scores are perfectly correlated. That is, each respondent score the exact same thing on T1 as they did on T2. A coefficient correlation of 0 indicates that the respondents’ scores at T1 were completely unrelated to their scores at T2; therefore the test is not reliable.
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So, how do we interpret the coefficients of stability that are between 1 and 0? The following guidelines can be used:
0.9 and greater: excellent reliability
Between 0.9 and 0.8: good reliability
Between 0.8 and 0.7: acceptable reliability
Between 0.7 and 0.6: questionable reliability
Between 0.6 and 0.5: poor reliability
Less than 0.5: unacceptable reliability
