Why are the point differences between predicted achievement and actual achievement that are needed for statistical significance so much smaller with the WIAT-IIUK as compared to the original WIAT (WOND, WORD and WOLD)?
Some of the difference in tabled values is the result of improved reliability of some of the WIAT-IIUK subtests (the better the reliability of the subtest, the smaller the difference required to reach statistical significance). However, the majority of the difference is attributable to a change in one aspect of the statistical formula used to calculate statistically significant differences between predicted and actual achievement scores. The change in formula is discussed in the WIAT-IIUK Updated Examiner’s Manual on page 157, where it is stated: “Users of the WIAT will note the similarity of formulas presented here but also that the calculations of extreme discrepancies (The Psychological Corporation, 1992, p. 188) are no longer included, in favour of the more widely used standard error of the residual.” “Calculation” or recognition of a “severe” discrepancy must still be based on statistical, as well as clinical significance. Base rate information, as included in the WIAT-IIUK Manual
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