When items exhibit different difficulties for different person groups this is called "item bias" or "differential item functioning", DIF. When the DIF is the same for all ability levels of the two groups this is termed Uniform DIF. Table 30 reports this. When DIF differs with ability levels, this is termed Non-Uniform DIF, NUDIF. This is discussed here.
1. Simple NUDIF reporting. In Table 30, specify both a person class column with DIF= and the number of ability strata to be reported, e.g., $MA2, which reports 2 levels of ability, the top and bottom half of the ability range.
Here is the plot:
The blue box flags an example of non-uniform DIF. It is for the "F", females, ability strata "2", the upper ability group, because MA2 means "measures ascending in two groups", so that group 1 is the lower ability group, and group 2 is higher ability group. The numbers are shown in Table 30.2:
Some person classes have extreme scores. These are shown in the green boxes, and are indicated by "<" for minimum extreme scores and ">" for maximum extreme scores. The plotted DIF measures are shown in the blue box, with the DIF size relative to the overall item difficulty level in the adjacent column.
The DIF t columns shows the t-statistic corresponding to the hypothesis test "this DIF Size was produced by chance". An overall test of the null hypothesis "this DIF is was produced by chance" is given by a chi-square composed of the sum of the squares of the t-statistics for the item, with (count of t-statistics - 1) d.f. Here the values is 4.6 with 3 d.f. with a probability of 0.20, so we cannot reject the hypothesis that this amount of DIF came about by chance.
2. For more insights, plot the non-uniform DIF ICCs.