The Base Rate Problem and Its Consequences for Interpreting Children's Ability Profiles.

Base rates refer to the proportion of a population that falls within a diagnostic category, either identifying an exceptionality (e.g., learning disability [LD], emotional disturbance [ED], or simply representing "normal" variation. This article familiarizes readers with the importance and meaning of base rates. It presents several univariate and multivariate base-rate procedures useful for identifying unusual IQ subtest variation. It compares the various base-rate procedures with the statistical significance-testing approach routinely used by psychologists. The mathematical superiority of one base-rate procedure is highlighted (i.e., the nonlinear multivariate baserate method), and its practical. and scientific benefits are discussed. The nonlinear multivariate base-rate method is used to address the more important question of whether subtest analysis has validity for differential decision making. Specifically, the nonlinear multivariate method is employed to determine whether children with LD eN ;: 925) and ED eN = 100) are more likely to show unusual subtest patterns than children from the nonnative sample of the Wechsler Intelligence Scale for Children-Third Edition eN;: 2,200). Results are discussed and recommendations are provided for improving futw'e research on subtest analysis. A golden anniversary is about to take place in the field of individual intelligence testing. The precipitating event occurred in 1949 when subtests were introduced on the newly created Wechsler Intelligence Scale for Children (WISC; Wechsler, 1949). Since then, literally hundreds of publications have promoted the analysis of children's subtest scores. This legacy of interpretive relevance continues to be reflected in textbooks on intelligence testing wherein, despite the recent presentation of some notable limitations and caveats, page-after-page remain devoted to the identification of unusual subtest patterns and the generation of inferential hypotheses about them (cf. Kamphaus, 1993; Kaufman, 1994; Sattier, 1992). The current article has several purposes. One is to familiarize readers with the term base rates. Another is to present one univariate, and two multivariate, base-rate methods for identifying unusual IQ subtest profiles. These procedures will be contrasted against the statistical significancetesting approach conunonly employed by psychologists. Serious limitations will be identified with (a) the statistical significance-testing approach, (b) the univariate base-rate approach, and ( c) the linear multivariate base-rate approach. An alternative method of interpretation will then be presented. This procedure employs nonlinear multivariate base rates as the mechanism for identifying unusual subtest profiles. This article will discuss the advantages of comparing subtest scores to these nonlinear multivariate taxonomies to obtain accurate base rates, and it will present a 10 subtest taxonomy developed recently for the standardization sample of the Wechsler IntelliAddress all correspondence concerning this article to Joseph J. Glutting, University of Delaware, School of Education, Department of Educational Studies, Willard Hall Education Building, Newark, DE 19716-2901. Copyright 1997 by the National Association of School Psychologists, ISSN 0279~6015.