USE OF GILLIAM ASPERGER S DISORDER SCALE IN DIFFERENTIATING HIGH AND LOW FUNCTIONING AUTISM AND ADHD 1

Transcription

1 Psychological Reports, 2011, 108, 1, Psychological Reports 2011 USE OF GILLIAM ASPERGER S DISORDER SCALE IN DIFFERENTIATING HIGH AND LOW FUNCTIONING AUTISM AND ADHD 1 SUSAN DICKERSON MAYES, SUSAN L. CALHOUN, AND MICHAEL J. MURRAY Department of Psychiatry Pennsylvania State College of Medicine JILL D. MORROW Commonwealth of Pennsylvania Department of Public Welfare Office of Developmental Programs, Harrisburg, PA SHIYOKO COTHREN, HEATHER PURICHIA Department of Psychiatry Pennsylvania State College of Medicine KIRSTEN K. L. YURICH The Vista Foundation, Hershey, PA JAMES N. BOUDER The Vista Foundation, Hershey, PA Summary. Little is known about the validity of Gilliam Asperger s Disorder Scale (GADS), although it is widely used. This study of 199 children with high functioning autism or Asperger s Disorder, 195 with low functioning autism, and 83 with Attention Deficit Hyperactivity Disorder (ADHD) showed high classification accuracy (autism vs ADHD) for clinicians GADS Quotients (92%), and somewhat lower accuracy (77%) for parents Quotients. Both children with high and low functioning autism had clinicians Quotients (M = 99 and 101, respectively) similar to the Asperger s Disorder mean of 100 for the GADS normative sample. Children with high functioning autism scored significantly higher on the Cognitive subscale than children with low functioning autism, and the latter had higher scores on the remaining subscales: Social Interaction, Restricted of Behavior, and Pragmatic Skills. Using the clinicians Quotient and Cognitive score, 70% of children were correctly identified as having high or low functioning autism or ADHD. The Gilliam Asperger s Disorder Scale (GADS) was designed to help professionals differentiate those persons who are likely to have Asperger s Disorder from those who do not (Gilliam, 2001, p. 6). Although the test is widely used, little is known about its psychometric properties. The only published data are found in the test manual and a study by Mayes, Calhoun, Murray, Morrow, Yurich, Mahr, et al. (2009). This study found that the clinicians GADS Asperger s Quotient identified children with high functioning autism or Asperger s Disorder with 92% accuracy, but 88% of children with low functioning autism also scored in the Asperger s 1 Address correspondence to Susan D. Mayes, Ph.D., Department of Psychiatry, H073, Milton S. Hershey Medical Center, 500 University Dr., Hershey, PA or (smayes@psu. edu). DOI / PR ISSN

2 4 S. D. MAYES, ET AL. Disorder range. These findings suggest that the GADS composite score (Asperger s Quotient) is not effective in distinguishing low from high functioning autism. The present purpose was to assess whether the GADS subscale scores (versus the Asperger s Quotient) can differentiate children with high and low functioning autism and if these two groups can be differentiated from children with ADHD. ADHD was selected as a clinical comparison group because ADHD is a common childhood disorder that, together with autism, comprises a large portion of referrals for child diagnostic evaluations. Children with ADHD and autism have many overlapping features, including inattention, overactivity, and mood and behavior problems (Mayes & Calhoun, 1999, 2007), early language delay (Mayes & Calhoun, 1999; Miniscalco, Hagberg, Kadesjo, Westerlund, & Gillberg, 2007), and difficulty with social skills (de Boo & Prins, 2007). These shared symptoms often complicate a differential diagnosis. Therefore, assessing whether GADS scores can differentiate between high functioning autism, low functioning autism, and ADHD is of clinical value. Method Participants This sample consisted of 195 children with diagnoses of autism and full scale IQs below 80 (low functioning autism), 199 children with diagnoses of autism or Asperger s Disorder and Full-scale IQs of 80 or above (high functioning autism), and 83 children with ADHD. The most commonly administered IQ tests were the Bayley Scales of Infant Development II Mental Scale (Bayley Mental Scale), Wechsler Preschool and Primary Scale of Intelligence III (WPPSI III), and Wechsler Intelligence Scale for Children IV (WISC IV). Children with high functioning autism and Asperger s Disorder were not differentiated because research shows that most, if not all, children with a clinical diagnosis of Asperger s Disorder actually meet DSM IV criteria for autism (Manjiviona & Prior, 1995; Szatmari, Archer, Fisman, Streiner, & Wilson, 1995; Eisenmajer, Prior, Leekam, Wing, Gould, Welham, et al., 1996; Miller & Ozonoff, 1997; Mayes, Calhoun, & Crites, 2001; Howlin, 2003; Mayes & Calhoun, 2003; Tryon, Mayes, Rhodes, & Waldo, 2006), as was also the case for the present sample. Children with autism or ADHD were consecutive referrals to the authors diagnostic clinics and were evaluated by licensed Ph.D. psychologists, board-certified child psychiatrists, or a board-certified developmental pediatrician using DSM IV criteria. Components of the diagnostic evaluation included a structured interview with the parent focusing on early history and current symptoms; behavior and autism rating scales completed by parents, teachers, and childcare providers; observations of the child during testing; and a review of early intervention, school, and

3 DIFFERENTIATING AUTISM AND ADHD 5 medical records. All children with autism had a score in the autistic range on the Checklist for Autism Spectrum Disorder (Mayes & Calhoun, 1999; Mayes, et al., 2009). Institutional Review Board approval and informed parental consent were obtained for this study. Demographic data for the sample are presented in Table 1. Children with high functioning and low functioning autism did not differ by age (t = 1.2, p =.23) or parental occupation, sex, and race (χ 2 < 6.3, p >.01). TABLE 1 Demographic Data For Children With High Functioning Autism, Low Functioning Autism, and Attention Deficit Hyperactivity Disorder (ADHD) High Functioning Autism Low Functioning Autism ADHD M SD M SD M SD Age, yr IQ Male, % Parent professional,* % White, % *One or both parents have a professional or managerial occupation. Gilliam Asperger s Disorder Scale The 32 items are rated on a 4-point scale (using anchors of 0: Never and 3: Frequently observed) by parents, teachers, psychologists, and others familiar with the test who have had sustained contact with the child (Gilliam, 2001). There are four subscales, Social Interaction, Restricted of Behavior, Cognitive, and Pragmatic Skills, yielding standard scores based on ratings for the 371 children 3 to 22 years of age with Asperger s Disorder in the normative sample. The sum of the four subscale standard scores is converted into an Asperger s Quotient using a table in the manual. The possible range of scores for the Quotient is 40 to 140, with a mean of 100 and standard deviation of 15 for children with Asperger s Disorder. Subscale scores have means of 10 and standard deviations of 3. According to the manual, If the subject s Asperger s Quotient is 80 or above, the person probably has Asperger s Disorder (Gilliam, 2001, p. 18). For the GADS standardization, parents, teachers, and personnel from school districts and treatment centers were contacted and asked to complete the test on individuals diagnosed as having Asperger s Disorder (Gilliam, 2001, p. 21). Diagnoses were not independently validated. Data reported in the manual show that Asperger s Quotients were significantly higher for the 371 individuals with diagnoses of Asperger s Disorder (M = 101) than for 50 individuals with Autistic Disorder (M = 72), 28 with other diagnoses including ADHD, mental retardation, emotional distur-

4 6 S. D. MAYES, ET AL. bance, and learning disabilities (M = 77), and 26 nondisabled individuals (M = 67). Using the Asperger s Quotient and the four subscale scores, discriminant analysis classified individuals with and without Asperger s Disorder with 83% accuracy. Procedure In this study, the GADS was completed by clinicians who had extensive experience and expertise with autism and ADHD, including two licensed Ph.D. psychologists, three board-certified child psychiatrists, one board-certified developmental pediatrician, and one certified school psychologist working at a school for children with autism. A subset of parents of these children independently completed the test for their children. For comparative purposes, clinicians also completed the Childhood Autism Rating Scale (Schopler, Reichler, & Renner, 1986) and the Checklist for Autism Spectrum Disorder (Mayes & Calhoun, 1999; Mayes, et al., 2009). Research supports the diagnostic validity of these two tests in identifying children with and without autism (Schopler, et al., 1986; Sevin, Matson, Coe, Fee, & Sevin, 1991; Mayes & Calhoun, 1999; Mayes, et al., 2001, 2009; Tryon, et al., 2006). Analyses Independent t tests and Cohen s d effect size were calculated to estimate the statistical and clinical significance of mean test scores of children with high functioning and low functioning autism. The significance of differences in frequency distributions of diagnostic groups was calculated using chi square. Pearson correlation coefficients estimated the relation of IQ with scores on the Gilliam Asperger s Disorder Scale and other autism tests. The most accurate cutoff score for the GADS subscale which best differentiated children with high and low functioning autism was selected by calculating the cutoff value which maximized overall classification accuracy and yielded approximately equivalent sensitivity and specificity. Binary logistic regression analysis was used to examine how well GADS subscale scores differentiated children with high functioning and low functioning autism. Percentages of classification accuracy, sensitivity, and specificity using clinicians and parents GADS Quotients and the recommended cutoff of 80 were calculated for children in each group. Results Mean Differences between Children with High and Low Functioning Autism Children with high functioning and low functioning autism did not differ on clinicians GADS Quotients. Children with high functioning autism had significantly higher mean clinicians scores on the Cognitive subscale, and children with low functioning autism had higher scores on Social Interaction, Restricted of Behavior, and Pragmat-

5 DIFFERENTIATING AUTISM AND ADHD 7 ic Skills. Similarly, children with high functioning and low functioning autism did not differ in parents Quotients, and children with high functioning autism had higher Cognitive scores than children with low functioning autism. Like clinicians scores, children with low functioning autism had higher mean parental ratings than children with high functioning autism on Social Interaction, Restricted of Behavior, and Pragmatic Skills, but the differences were not significant. Mean clinicians and parents GADS Quotients were considerably lower for children with ADHD than for children with autism. A few children in each group scored unusually high or low, resulting in a large range of overlapping scores. TABLE 2 GADS Scores for Children with High Functioning Autism (HFA), Low Functioning Autism (LFA), and Attention Deficit Hyperactivity Disorder (ADHD) GADS Quotient Social Interaction Restricted Cognitive Pragmatic Skills M SD Range M SD M SD M SD M SD Clinicians scores HFA LFA ADHD Parents scores HFA LFA ADHD Note. Clinicians scores: comparison of HFA vs LFA Quotients, t = 0.9, p =.36, d = 0.1; HFA > LFA Cognitive, t = 9.2, p <.0001, d = 1.0; LFA > HFA Social Interaction, Restricted of Behavior, and Pragmatic Skills, ts = 3.6, 5.4, and 4.3, all p <.0001, ds = 0.4, 0.8, and 0.3, respectively. Parents scores: HFA vs LFA nonsignificant Quotients, t = 0.3, p =.78, d = 0.1; HFA > LFA Cognitive, t = 4.0, p <.0001, d = 0.9; LFA vs HFA nonsignificant Social Interaction, Restricted of Behavior, and Pragmatic Skills, ts = 1.3, 1.8, and 0.3, p >.07, ds = 0.3, 0.3, and 0.0, respectively. Cutoff Score Differentiating High and Low Functioning Autism Clinicians and parents Cognitive scores differed more than those on other subscales for children with high and low functioning autism. The clinicians cutoff score which maximized classification accuracy and equalized sensitivity and specificity was 7. Sixty-eight percent of the children with high functioning autism had Cognitive scores of 7 or higher, and 73% of the children with low functioning autism had scores below 7. The most accurate cutoff was also 7 for parents Cognitive scores, which correctly identified 65% of the children with high functioning autism and 69% with low functioning autism. Logistic Regression Analysis Using clinicians GADS subscale scores and logistic regression analy-

6 8 S. D. MAYES, ET AL. sis, 75% of children with high functioning autism and 75% with low functioning autism were correctly identified (Table 3). The Cognitive subscale had a large positive weight, the Restricted and Pragmatic Skills subscales had small negative weights, and Social Interaction had a weight of 0. TABLE 3 Logistic Regression Analysis Using GADS Subscale Scores to Differentiate Children With High Functioning Autism and Low Functioning Autism β OR 95%CI SE p Social Interaction , Restricted , Cognitive , <.001 Pragmatic Skills , <.001 Constant Note. Nagelkerke R 2 =.40, Hosmer and Lemeshow test χ 2 = p =.24. Pearson Correlations for IQ and Scores on the GADS and Other Autism Tests For children with high and low functioning autism (Table 4), the clinicians Cognitive score was significantly positively correlated with IQ, and the remaining Social Interaction, Restricted of Behavior, and Pragmatic Skills scores were significantly negatively correlated with IQ. The parents Cognitive score and IQ were also significantly positively correlated. The remaining parents scores were negatively correlated with IQ, but the association was not significant. Score TABLE 4 Pearson Correlations for IQ with GADS Scores For Children With High Functioning Autism and Low Functioning Autism GADS Quotient Social Interaction Restricted Cognitive Pragmatic Skills r p r p r p r p r p Clinician < < <.001 Parent < For children with high and low functioning autism (Table 5), the clinicians GADS Social Interaction, Restricted, and Pragmatic Skills scores correlated significantly and positively with each other and with scores on two other autism tests, the Childhood Autism Rating Scale (Schopler, et al., 1986) and the Checklist for Autism Spectrum Disorder (Mayes & Calhoun, 1999; Mayes, et al., 2009). In contrast, GADS Cognitive scores correlated significantly and positively with scores on only Pragmatic Skills and showed either inverse or nonsignificant relations with the other GADS subscales and autism tests. Similarly, for the high

7 DIFFERENTIATING AUTISM AND ADHD 9 TABLE 5 Pearson Correlations For Clinicians GADS Subscale Scores With Scores on Other Autism Tests For Children With High Functioning Autism and Low Functioning Autism Restricted Cognitive Pragmatic Skills CARS 1 CASD 2 r p r p r p r p r p Social Interaction.48 < < < <.001 Restricted < < <.001 Cognitive.25 < < Pragmatic Skills.28 < <.001 CARS.53 < Childhood Autism Rating Scale. 2 Checklist for Autism Spectrum Disorder. functioning autism group alone, Cognitive was the only score that did not significantly and positively correlate with scores on the other GADS subscales or autism tests, and it was the only score significantly and positively correlated with IQ. All other scores correlated significantly and positively with each other and negatively with IQ. Internal consistency for the GADS subscale scores was lower for children with high functioning autism (Cronbach s α =.58) than for children with low functioning autism (Cronbach s α =.71). Classification Accuracy for Children with High and Low Functioning Autism and ADHD Using the GADS Quotient and the recommended cutoff of 80, percentages of sensitivity and specificity for differentiating children with autism from children with ADHD were high and were similar for children with high and low functioning autism (Table 6). Overall classification accuracy ranged from 90 to 93% for clinicians Quotients and 80 to 81% for parents Quotients. GADS scores were less accurate in differentiating between high and low functioning autism. Clinicians Quotients and Cognitive scores were used together to classify children with high and TABLE 6 Percentages of Classification Accuracy For Children With High Functioning Autism, Low Functioning Autism, and Attention Deficit Hyperactivity Disorder (ADHD) Using the GADS Quotient Cutoff Score of 80 Comparison Overall Accuracy Sensitivity Specificity Clinicians scores High functioning autism vs ADHD Low functioning autism vs ADHD Parents scores High functioning autism vs ADHD Low functioning autism vs ADHD

8 10 S. D. MAYES, ET AL. low functioning autism and ADHD. The following formulas and designations were applied: GADS Quotient 80 and Cognitive subtest 7 for high functioning autism, GADS Quotient 80 and Cognitive < 7 for low functioning autism, and GADS Quotient < 80 for nonautistic. Using these formulas, 68% of the 199 children with high functioning autism were correctly identified, 62% of the 195 children with low functioning autism were identified, and 95% of the 83 children with ADHD were identified as not having autism, yielding an overall classification accuracy of 70%. Discussion The GADS is a widely used yet little researched test. Aside from data reported in the manual and one published study (Mayes, et al., 2009), there are no studies of test validities or reliabilities using the GADS. The present goal was to assess whether scores on the GADS subscales could differentiate children with high and low functioning autism and differentiate these children from children with ADHD. The subtest on which high and low functioning children with autism differed the most was Cognitive. Of children with high functioning autism, 68% had clinicians GADS Quotients 80 and Cognitive scores 7, and 62% of children with low functioning autism had clinicians GADS Quotients 80 and Cognitive scores < 7. Of children with ADHD, 95% had Quotients below 80. This resulted in 70% overall classification accuracy for these three groups. Although this accuracy is substantial and the pattern of GADS scores may be helpful diagnostically, a classification accuracy of 70% is not high enough to justify using GADS scores alone to identify high functioning versus low functioning autism. Further, classification accuracy is likely to be lower if the equation derived in this study is applied to a different sample of children. Children with high functioning autism had significantly higher mean Cognitive scores than children with low functioning autism, and children with low functioning autism had significantly higher scores on the other subscales (Social Interaction, Restricted of Behavior, and Pragmatic Skills) than children with high functioning autism. This suggests that the two groups differ in severity of autistic symptoms (represented by the Social Interaction, Restricted of Behavior, and Pragmatic Skills scores) and IQ (reflected in the Cognitive score). IQ was significantly positively correlated with Cognitive scores and negatively correlated with Social Interaction, Restricted of Behavior, and Pragmatic Skills scores. Other studies have also found that autism severity increases with decreasing IQ (Sevin, Matson, Coe, Love, Matese, & Benavidez, 1995; Myhr, 1998; Prior, Eisenmajer, Leekam, Wing, Gould, Ong, et al., 1998; Miller & Ozonoff, 2000; Mayes & Calhoun, 2004; Mayes,

9 DIFFERENTIATING AUTISM AND ADHD 11 et al., 2009) and that children with low functioning autism and high functioning autism or Asperger s Disorder differ primarily in autism severity and IQ (Myhr, 1998; Miller & Ozonoff, 2000; Ozonoff, South, & Miller, 2000; Mayes & Calhoun, 2004). Therefore, present data and other findings do not support the interpretation that low functioning autism and high functioning autism or Asperger s Disorder are separate disorders, because individuals with any disorder (e.g., ADHD or depression) can differ in IQ and symptom severity. For children with autism, the clinicians Cognitive scores were more highly correlated with IQ than with the other GADS subscales and were negatively correlated with scores on two other autism tests, the Childhood Autism Rating Scale (Schopler, et al., 1986) and the Checklist for Autism Spectrum Disorder (Mayes & Calhoun, 1999; Mayes, et al., 2009). This suggests that Cognitive may measure cognition more than autism. These findings are different from data in the GADS manual showing that Cognitive scores correlated significantly and positively with scores on the three other GADS subscales in the normative sample of children with Asperger s Disorder. Further research is needed to explore this discrepancy in findings. Groups of children with high and low functioning autism in this study did not differ from each other on clinicians GADS Asperger s Quotients (M = 99 and 101, respectively). For the sample of children with high and low functioning autism or ADHD, the clinicians Quotient cutoff score of 80 resulted in similarly high classification accuracy for children with high functioning autism (93%) and low functioning autism (90%), suggesting that the GADS Quotient may identify high and low functioning autism equally well, but may not differentiate the two. Data suggest that clinicians GADS Quotients were more accurate than parents Quotients in identifying children with and without autism. Although the GADS manual states that the GADS may be completed by parents or professionals, 253 of the 371 raters in the GADS normative study were parents, and only 10 raters were psychologists. More research is needed to assess whether GADS scores differ as a function of the rater and what types of raters produce the most accurate GADS scores. Children with high functioning autism earned clinicians GADS Quotients and Social Interaction, Restricted of Behavior, and Pragmatic Skills subscale scores consistent with those for children with a diagnosis of Asperger s Disorder in the GADS normative sample (Gilliam, 2001). However, other scores differed from those reported in the GADS manual. Children with low functioning autism earned a mean GADS Quotient of 101, which compares with a mean of 72 for the 50 individuals diagnosed with Autistic Disorder in the GADS standardization sample

10 12 S. D. MAYES, ET AL. (Gilliam, 2001). Children with high functioning autism had a mean Cognitive subscale score of 8, which is considerably lower than the mean of 10 for children with Asperger s Disorder in the GADS normative sample. Given the paucity of published data on the GADS and these conflicting findings, further research is required. A major limitation of the present study is the absence of a comparison group with mental retardation and no autism, which needs to be included so that studies are representative of children evaluated in diagnostic clinics. In summary, classification accuracy for the clinicians and parents GADS scores combined with diagnostic validity data from the GADS manual (Gilliam, 2001) and a previous study (Mayes, et al., 2009) are encouraging and support the potential utility of GADS scores in providing clinicians with information to help screen for and diagnose autism. The GADS and other autism tests, such as the Childhood Autism Rating Scale (which has excellent diagnostic validity for low functioning autism; Schopler, et al., 1986, Mayes, et al., 2009) and the Checklist for Autism Spectrum Disorder (which has diagnostic validity for identifying children with both high and low functioning autism, Mayes, et al., 2009), offer a simple and cost effective means of collecting standardized, reliable, valid, and norm-referenced data from multiple sources and settings to aid the diagnostic process. Information from these tests completed by clinicians, parents, teachers, and childcare providers can be combined with data from parental interviews (focusing on early history and current symptoms), standardized observations of the child, and review of records to assist clinicians in evaluating whether the children s behavior meets diagnostic criteria for autism. REFERENCES de Boo, G. M., & Prins, P. J. M. (2007) Social incompetence in children with ADHD: possible moderators and mediators in social skills training. Clinical Psychology Review, 27, Eisenmajer, R., Prior, M., Leekam, S., Wing, L., Gould, J., Welham, M., & Ong, B. (1996) Comparison of clinical symptoms in autism and Asperger s disorder. Journal of the American Academy of Child and Adolescent Psychiatry, 35, Gilliam, J. E. (2001) Gilliam Asperger s Disorder Scale manual. Austin, TX: Pro-Ed. Howlin, P. (2003) Outcome on high-functioning adults with autism with and without early language delays: implications for the differentiation between autism and Asperger syndrome. Journal of Autism and Developmental Disorders, 33, Manjiviona, J., & Prior, M. (1995) Comparison of Asperger syndrome and highfunctioning autistic children on a test of motor impairment. Journal of Autism and Developmental Disorders, 25, Mayes, S. D., & Calhoun, S. L. (1999) Symptoms of autism in young children and correspondence with the DSM. Infants and Young Children, 12, Mayes, S. D., & Calhoun, S. L. (2003) Relationship between Asperger syndrome and high-functioning autism. In M. Prior (Ed.), Learning and behavior problems in Asperger syndrome. New York: Guilford. Pp

11 DIFFERENTIATING AUTISM AND ADHD 13 Mayes, S. D., & Calhoun, S. L. (2004) Influence of IQ and age in childhood autism: lack of support for DSM IV Asperger s disorder. Journal of Developmental and Physical Disabilities, 16, Mayes, S. D., & Calhoun, S. L. (2007) Learning, attention, writing, and processing speed in typical children and children with ADHD, autism, anxiety, depression, and oppositional-defiant disorder. Child Neuropsychology, 13, Mayes, S. D., Calhoun, S. L., & Crites, D. L. (2001) Does DSM IV Asperger s disorder exist? Journal of Abnormal Child Psychology, 29, Mayes, S. D., Calhoun, S. L., Murray, M. J., Morrow, J. D., Yurich, K. K. L., Mahr, F., Cothren, S., Purichia, H., Bouder, J. N., & Petersen, C. (2009) Comparison of scores on the Checklist for Autism Spectrum Disorder, Childhood Autism Rating Scale (CARS), and Gilliam Asperger s Disorder Scale (GADS) for children with low functioning autism, high functioning autism, Asperger s Disorder, ADHD, and typical development. Journal of Autism and Developmental Disorders, 39, Miller, J. N., & Ozonoff, S. (1997) Did Asperger s cases have Asperger disorder? A research note. Journal of Child Psychology and Psychiatry, 38, Miller, J. N., & Ozonoff, S. (2000) The external validity of Asperger disorder: lack of evidence from the domain of neuropsychology. Journal of Abnormal Psychology, 109, Miniscalco, C., Hagberg, B., Kadesjo, B., Westerlund, M., & Gillberg, C. (2007) Narrative skills, cognitive profiles, and neuropsychiatric disorders in 7 8-year-old children with late developing language. International Journal of Language and Communication Disorders, 42, Myhr, G. (1998) Autism and other pervasive developmental disorders: exploring the dimensional view. Canadian Journal of Psychiatry, 43, Ozonoff, S., South, M., & Miller, J. N. (2000) DSM IV-defined Asperger syndrome: cognitive, behavioral and early history differentiation from high-functioning autism. Autism, 4, Prior, M., Eisenmajer, R., Leekam, S., Wing, L., Gould, J., Ong, B., & Dowe, D. (1998) Are there subgroups within the autistic spectrum? A cluster analysis of a group of children with autistic spectrum disorders. Journal of Child Psychology and Psychiatry, 39, Schopler, E., Reichler, R. J., & Renner, B. R. (1986) The Childhood Autism Rating Scale (CARS). Los Angeles: Western Psychological Services. Sevin, J. A., Matson, J. L., Coe, D., Fee, V. E., & Sevin, B. M. (1991) A comparison and evaluation of three commonly used autism scales. Journal of Autism and Developmental Disorders, 21, Sevin, J. A., Matson, J. L., Coe, D., Love, S. R., Matese, M. J., & Benavidez, D. A. (1995) Empirically derived subtypes of pervasive developmental disorders: a cluster analytic study. Journal of Autism and Developmental Disorders, 25, Szatmari, P., Archer, L., Fisman, S., Streiner, D. L., & Wilson, F. (1995) Asperger s syndrome and autism: differences in behavior, cognition, and adaptive functioning. Journal of the American Academy of Child and Adolescent Psychiatry, 34, Tryon, P. A., Mayes, S. D., Rhodes, R., & Waldo, M. (2006) Can Asperger s disorder be differentiated from autism using DSM IV criteria? Focus on Autism and Other Developmental Disabilities, 21, 2-6. Accepted December 20, 2010.