Monday, January 31, 2011

IQ test "practice effects"

A practice effect is a major psychometric issue in many Atkins cases, given that both the state and defense often test the defendant with the same IQ battery (most often a Wechsler), and often within a short test-retest interval. Click here to view all ICDP posts that mention practice effects.

Dr. Alan Kaufman has summarized the majority of the literature on practice effects on the Wechslers. He published an article in The Encyclopedia of Intelligence (1994; Edited by Robert Sternberg) that summarized the research prior to the third editions of the Wechsler scales. That article is available on-line (click here).

The most recent summary of the contemporary Wechsler practice effect research is in Lichtenberger and Kaufman (2009) Essentials of WAIS-IV Assessment (p. 306-309). The tables and text provide much about WAIS-IV and some about WAIS-III. The best source for WAIS-III is Kaufman and Lichtenberger, Assessing Adolescent and Adult Intelligence (either the 2002 second edition or the 2006 third edition), especially Tables 6.5 and 6.6 (2006 edition). Below are a few excerpts from the associated text from the 2006 edition

"Practice effects on Wechsler's scales tend to be profound, particularly on the Performance Scale" (p. 202)

"predictable retest gains in IQs" (p.202)

"On the WAIS-III, tests with largest gains are Picture Completion, Object Assembly, and Picture Arrangement"

"Tests with smallest gains are Matrix Reasoning (most novel Gf test), Vocabulary and Comprehension

Block Design improvement most likely due to speed variance--"on second exposure subjects may be able to respond more quickly, thereby gaining in their scores" (p. 204)

One year interval results in far less pronounced practice effects (p. 208).

"The impact of retesting on test performance, whether using the WAIS-III, WAIS-R, other Wechsler scales, or similar tests, needs to be internalized by researchers and clinicians alike. Researchers should be aware of the routine and expected gains of about 2 1/2 points in V-IQ for all ages between 16 and 89 years. They should also internalize the relatively large gain on P-IQ for ages 16-54 (about 8 to 8 1/2 points), andn the fact that this gain in P-IQ swindles in size to less than 6 points for ages 55-74 and less than 4 points for ages 75-889" (p. 209).

"Increases in Performance IQ will typically be about twice as large as increases in Verbal IQ for individuals ages 16 to 54" (p. 209)

Finally, the latest AAIDD manual provides professional guidance on the practice effect.

"The practice effect refers to gains in IQ scores on test of intelligence that result from a person being retested on the same instrument" (p. 38)

"..established clinical practice is to avoid administering the same intelligence test within the same year to the same individual because it will often lead to an overestimate of the examinee's true intelligence" (p. 38).

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