2 times the standard deviation!
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95% is within 2 standard deviations of the mean.
Square the standard deviations, subtract/add them and calculate the square root of the subtraction/sum. StDV=sqrt (StDvA^2+StDvB^2)
I believe the standard deviations are measured from the median, not the mean.1 Standard Deviation is 34% each side of median, so that is 68% total.2 Standard Deviations is 48% each side of median, so that is 96% total.
The sum of standard deviations from the mean is the error.
The mean for the WISC, like the WAIS, is 100. The deviations from 100, or standard deviations, are 15.