Wednesday, April 07, 2010

Psychometric PS to Johnston v Florida (2010) denied appeal re: new WAIS-IV scores

This is a follow-up to my brief comments yesterday regarding the Johstone v Fl (2010) denied MR/ID appeal of two days ago.

As mentioned in the decision and my blog comment, the WAIS-III/WAIS-IV tests correlated .94 in a study reported in the WAIS-IV technical manual.  This is a very high correlation...but does NOT mean that the two tests should be expected to provide identical IQ scores.  I discuss these issues in a prior IAP AP101 report.

The tests have different norm dates and thus, the later version (WAIS-IV) would be expected to provide a lower score based on the Flynn effect.  More importantly, as reported in the IAP AP101 report, when one calculates the standard deviation of the difference score (see page 6 of that report) for a correlation of .94, the resulting value is 5.2 (round to 5 for ease of discussion).  This means that, on average, the WAIS-III/WAIS-IV (even if highly correlated at the .94 level) would in the general population be expected to display a range of difference scores from -5 to +5...or a range of 10 IQ 68% of the population.  Please review that prior report for further explanation and discussion.

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