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Yet another article giving a formula without explaining all the variables. Pi and Qi would seem to be the proportion of people getting an item correct or incorrect as described here [] . It might also be helpful to indicate the variance and associated sum of squares are applied to raw scores with N being the number of people in the sample (not number of test items). —Preceding unsigned comment added by 126.96.36.199 (talk) 15:11, 22 October 2010 (UTC)