Long-term Stability of the Wechsler Intelligence Scale for Children- Third Edition among Students with Disabilities

Long-term stability of the Wechsler Intelligence Scale for Children-Third Edition (WISC-III) was investigated for children with specific learning disability (SLD), serious emotional disability (SED), and mental retardation (MR). Participants were 522 students from 33 states twice evaluated for special education eligibility over a mean test-retest interval of 2.87 years. There were no differential effects of disability groups on long-term stability coefficients. Stability coefficients for Full Scale IQ (FSIQ) scores were acceptable for all three disability groups. Of the global IQ and Index scores, only the Freedom from Distractability Index, Processing Speed Index, and Verbal IQ-Performance IQ discrepancy score stability coefficients were inadequate. Subtest stability was also inadequate. Mean changes from first testing to second testing for IQ and Index scores were not significant and the two significant subtest changes were not clinically meaningful due to small effect sizes. Individual change scores revealed that only the FSIQ was sufficiently stable for use with individual students with SLD, SED, or MR. Results extended those of Canivez and Watkins (1998, 1999) supporting long-term stability for the WISC-III. to examine mean changes to supplement correlational analyses in order to investigate level as well as pattern (rank order) of relationships. Additionally, individual score changes from one testing session to another also have been utilized by researchers as another indicator of stability (Canivez & Watkins, 1998, 1999; Elliott et al., 1985; Stavrou, 1990). The Wechsler Scales are the most frequently used measures of cognitive abilities This research was supported, in part, by a 1995-1996 Eastern Illinois University Faculty Development Grant and a 1995-1996 Pennsylvania State University College of Education Alumni Society Faculty Research Initiation Grant. The authors wish to express their gratitude to the 145 school psychologists who generously responded to our request for WISC-III data. They also thank Tim Runge, Lisa Samuels, and Daniel Heupel for assistance