Specialists in testing and scales construction believe that the value of the reliability coefficient found by test-retest method is usually noticed to be higher than the value of that found by Alpha cronbach method. Is there a scientific justification for this differenc?
Actually, mathematically, Bareinbom (1957) showed that the two formulae were paradoxical. Higher internal reliability meant lower retest reliability.
Hope this helps,
Matt
I agree with Matthew and i think it is commonly the case that the alpha coefficient is larger than the test-retest correlation.
If the assumptions underlying both reliability measures were met, i.e. essential tau-equivalence for alpha and parallel measures for retest reliability, both measures would be identical. However, the assumptions are seldom tested, and violating the assumptions leads to different values. One reason for a low rest-reliability is the time-lag: With increasing time interval between the measurement time-points the correlation will decrease because of occasion-specific influences. This correlation is often treated as a reliability coefficient, which is generally not correct because the assumptions for this coefficient were not met.
HTH, Karin
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