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Hypothetical Percent of Population Obtaining Differences between VCI, PRI and FSIQ scores

      VCI vs FSIQ   PRI vs FSIQ      
                 
  50   5.67   5.48   25  
  25   9.45   9.13   12.5  
  20   10.52   10.16   10  
  10   13.56   13.10   5  
  5   16.10   15.56   2.5  
  2   19.14   18.49   1  
  1   21.20   20.48   .5  
  .1   27.11   26.19   .05  
                 

To use this table:

Find the discrepancy that is just less than the one obtained by the examinee.  The left-most, bold column gives, in the same row, the percentage of the population obtaining discrepancies as large as or larger than the located discrepancy.  [If you are interested only in discrepancies in one direction, e.g., only VCI higher than FSIQ, use the bold percentages in the right-most column.]

A VCI - FSIQ discrepancy of 15 points would be found in approximately 5% to 10% of the population (16.10 = 5%; 13.56 = 10%).

The method used to compute the discrepancy between scales that reflect the percentage of the population obtaining the discrepancy is:

Discrepancy = SD * z * squareroot(2 - 2*rxy)

Discrepancy = standard deviation of the tests times the selected z value times the square root of two minus twice the correlation between the two tests.

For example, the discrepancy between the WISC-IV VCI and FSIQ that represents 5% of the population (z = 1.96) is:

15 * (1.96) * square root(2 - 2*0.85) =

15 * 1.96 * square root (2 - 1.7) =

29.4 * squareroot(0.3) =

29.4 * 0.55 =

16.1