Reliability and Validity of the Pervasive Developmental Disorders Rating Scale and the Gilliam Autism Rating Scale

The psychometric properties of the Pervasive Developmental Disorders Rating scale (Eaves, 2003) and the Gilliam Autism Rating Scale (Gilliam, 1995) were investigated in this study. One hundred thirty-four individuals with autism, other pervasive developmental disorders, or conditions frequently confused with autism participated in the study. The results indicated that, with one exception, the reliability of the scores from both instruments met or exceeded standards for use in screening decisions. The reliability of the total scores from both instruments exceeded .90. Validity coefficients computed between the two sets of scores indicated that the instruments measured similar constructs (e.g., rpddrs total x gars total .84). The scores from both instruments discriminated between children with autism and children who were not autistic to a statistically significant degree. The purpose of this research was to examine the reliability and validity of two screening instruments: the Gilliam Autism Rating Scale (GARS; Gilliam, 1995), and the Pervasive Developmental Disorder Rating Scale (PDDRS; Eaves, 2003). The GARS is purported to identify individuals with autistic disorder, one of five pervasive developmental disorders (PDD) defined in the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision (DSM-IV-TR; American Psychiatric Association, 2000). The PDDRS purports to identify individuals with PDD. In an effort to estimate the reliability and validity of the GARS and PDDRS, we employed four sets of analyses. First, Salvia and Ysseldyke (2004) have established critical reliability values for specific decisions. For making eligibility and classification decisions, these authors recommended a minimum reliability coefficient of .90. For screening decisions, they recommended a minimum reliability coefficient of .80. Although the GARS and PDDRS are not recommended for eligibility decisions by their respective authors, they can reasonably be held to a standard of r .80. A standard way of estimating the validity of an instrument is to compare the correlations of its scores with another instrument designed to serve the same, or a similar purpose. Because the GARS and PDDRS do purport to serve similar purposes, we established as a second objective of our research to compute validity coefficients between sets of scores obtained from the GARS and the PDDRS on the same participants. Although it does not provide compelling evidence, it is reasonable to expect such instruments as the GARS and PDDRS to discriminate between autistic-PDD groups and non autistic-non PDD groups. It was our third objective to test this reasonable expectation. While positive results support the validity of instrument, a more severe test is the determination of whether or not the instrument successfully classifies individuals. Our fourth set of analyses sought to assess the classification accuracy of the GARS and PDDRS for individuals. This included the estimation of sensitivity and specificity for the two instruments on a sample comprised of participants with autistic disorder, Asperger’s disorder, pervasive developmental disorder-not otherwise specified, Correspondence concerning this article should be addressed to Ronald C. Eaves, Department of Rehabilitation and Special Education, 1228 Haley Center, Auburn University, AL 36849. Email: [email protected] Education and Training in Developmental Disabilities, 2006, 41(3), 300–309 © Division on Developmental Disabilities 300 / Education and Training in Developmental Disabilities-September 2006 and participants with disabilities who were also suspected of having a PDD. We considered these analyses to pose the most severe test of the instruments. The following questions were addressed in this study: 1. To what extent do the GARS and PDDRS measure their respective dimensions accurately? 2. To what extent do the GARS and PDDRS measure the same constructs? 3. Do the GARS and PDDRS discriminate between groups of individuals with different diagnoses? 4. To what extent do the GARS and PDDRS classify individuals with different diagnoses accurately?