DIF Plot - Table 30.2

DIF Measure (diff.)

reports the difficulty = (diff.) of the item for each person classification: DIF "local absolute" Measure = DIF "relative" Size + Overall "baseline" item difficulty

* indicates the baseline measures (no DIF), corresponding to the zero line (x-axis) in the DIF Size plot.

DIF Size (diff.)

reports the size of the item DIF for the person-classification relative to the overall "baseline" item difficulty.

DIF t-value  (diff.)

reports a simple approximate t-test of the item DIF against the overall item difficulty. The t-statistic is the statistical probability of the DIF size relative to its measurement error expressed as an approximate unit-normal deviate. A critical values is usually ±2. This tests the hypothesis that the DIF size can be attributed to measurement error.

For a test of one group against another group, the joint t-value is usually around 70% of the difference between their t-values on the plot.

DIF Average Score-point Difference

displays the average difference between the observed response and the expected response for each person CLASS on each item.

Average Score-points

displays the average of the observed responses for each person CLASS on each item. Here the items are in Measure order: easiest items to hardest items.

DIF Worksheet

Sort the section of the worksheet to change the left-to-right order of the items in the plots.

Edit the worksheet to change the words shown in the plots.

* is the baseline column for "everyone". These are the item difficulties on Table 14.