Two-tailed versus one-tailed base rates of discrepancy scores in the WAIS-III

Tables included in the WAIS-III report the frequency of discrepancies between IQ and index scores independent of the directionality of the score. If an examiner does not have any a priori hypothesis about which skill may be more developed and which skills may be weaknesses, it is appropriate to use these tables as they are. In this case the examiner would be looking for unusually large discrepancies between scores irrespective of direction. When an examiner has a hypothesis about which skills may be weaknesses and strengths for an individual, the frequencies that are based upon an absolute value of the discrepancy will cause examiners to overestimate the frequency of the occurrence of the discrepancy score in question. Sattler and Ryan (1998) and Tulsky, Zhu, and Vasquez (1998) suggested dividing the frequencies reported in the WAIS-III tables in half to obtain the correct base rate. This suggestion is tested in this paper. New observed frequency tables were derived from the WAIS-III and WMS-III standardization samples and these frequencies were compared against the estimated frequencies using the method described by Sattler and Ryan (1998) and Tulsky et al. (1998). The differences between these two methods were calculated and are, for the most part, insignificant. In light of the similarity between the methods, the implications of using observed frequencies versus estimated frequencies is discussed.