Intrasubtest scatter (ISS) is a tendency toward unusual patterns in responses to test items (Wechsler, 1958). This is manifested as misfit to the Rasch model. The items on the Wechsler scales are ordered from easiest to most difficult using data from the standardization sample. Inconsistent patterns consist of getting some of the easier items wrong while getting the harder items right or a tendency toward isolated failures within long runs of correct responses.
Neuropsychological assessment and diagnosis often involves the detection and measurement of off-task or unusual responses in individuals suspected of cognitive dysfunction. An unusual item response pattern requiring neuropsychological interpretation occurs when an individual of high ability demonstrates a tendency to fail easy items. This response pattern may indicate cognitive inefficiency, difficulty with recall of specific information, or variable levels of arousal/attention. Almost all neuro-diagnostic interpretation of ISS has been conducted on adult populations using the Wechsler Adult Intelligence Scale (WAIS) or its revision (WAIS-R). In a review of their own and previous research, Mittenberg et al. (1989) concluded that scatter is associated with diffuse rather than focal neurological damage which results in either random loss of stored information or variable levels of arousal or attention. Surprisingly, the diagnostic utility of ISS has received comparatively little investigation in spite of frequently repeated recommendations that item scatter be interpreted as a qualitative indicator of cognitive dysfunction.
Our own study investigated the clinical utility of ISS in children experiencing attentional and information processing difficulties. The WISC-R item responses of 100 children who had received cranial irradiation treatment (with its risk of brain damage) for acute lymphoblastic leukaemia (ALL) were compared with those of 100 healthy children. The degree to which subjects in each of the two groups responded to items as predicted by an estimate of their ability was analyzed with a Rasch partial credit computer program (Adams & Khoo, 1993). The person Infit mean-square was the critical indicator.
Since brain-damage affects performance differentially, variation in subject ability across the eight Wechsler subtests was able to accurately classify 60% of the ALL cases and 70% of the controls. ISS subject misfit statistics derived from the eight subtests were able to classify correctly 61% of the ALL patients and 69% of the controls. The combination of ability variation across subtests and misfit within subtests correctly classified 71% of the ALL cases and 76% of the controls. The fact that the scatter misfit statistics were as successful as subtest ability variation in identifying subject type supports the conclusion that ISS is diagnostically useful for cases with relatively normal intellectual profiles.
With the widespread and increasing use of large scale testing programs for school-age students and with the computerized scoring of these test protocols, item scatter scores (misfit statistics) can become a screening measure for students with unrecognized cognitive dysfunction. Our research indicates that unusual response patterns on reading, spelling and arithmetic tests will prove to be useful pathognomic signs. A useful next step would be an investigation of the usefulness of item response variability on the Wide Range Achievement Test (WRAT, Jastak & Jastak, 1992, RMT 8:4 403-404) as a pathognomic sign.
Please see Psychological Assessment Resources for more information on the WRAT.
Adams, R.J. & Khoo, S. 1993. Quest, The Interactive Test Analysis System. Melbourne: ACER
Jastak, J. & Jastak, S. (1984) The Wide Range Achievement Test. Wilmington, DE: Jastak
Mittenberg, W., Thompson, G.B., Schwartz, J.A., Ryan, J.J., & Levit, R. (1991) Intellectual loss in Alzheimer's dementia and WAIS-R intrasubtest scatter. Journal of Clinical Psychology, 19, 420-423
Wechsler, D. (1958) Measurement and Appraisal of Adult Intelligence (4th ed.) Baltimore: Williams and Wilkins
Intrasubtest scatter in paediatric neuropsychology. Godber T, Anderson V. Rasch Measurement Transactions, 1996, 9:4 p.469
Please help with Standard Dataset 4: Andrich Rating Scale Model
|Rasch Measurement Transactions (free, online)||Rasch Measurement research papers (free, online)||Probabilistic Models for Some Intelligence and Attainment Tests, Georg Rasch||Applying the Rasch Model 3rd. Ed., Bond & Fox||Best Test Design, Wright & Stone|
|Rating Scale Analysis, Wright & Masters||Introduction to Rasch Measurement, E. Smith & R. Smith||Introduction to Many-Facet Rasch Measurement, Thomas Eckes||Invariant Measurement: Using Rasch Models in the Social, Behavioral, and Health Sciences, George Engelhard, Jr.||Statistical Analyses for Language Testers, Rita Green|
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