Figure 1

Item Response Function for the WOMAC Pain subscale (pre-data) and Person-Item-Threshold map. The Figure displays both the Item Response Function for the WOMAC Pain subscale and the Person-Item threshold map to illustrate the location of the item thresholds (clustered closely together along the bottom half of the lower part of the diagram) and the consequences for the response function of the scale (upper half of the diagram). The y-axes display the raw scores (top y-axis) which range from 0 to 250 as we divided the VAS scores by half for the analysis and the subscale contains five items, and the frequencies of item thresholds and participants (bottom y-axes). The Figure also shows the location of study participants along the construct of Pain. Data for this figure represent the unbiased person estimates derived from Analysis 2 (see also Table 1) which combined biased items 2 and 4 into a testlet and left the remaining items are unchanged.