Fig. 5

Exemplar sample-to-scale targeting plot—RMT analysis results for the Physical Fatigue scale. This figure depicts the person-item threshold distributions for the MG Symptoms PRO Physical Fatigue scale score, with persons (sample) distribution on top and scale item threshold distribution plotted on the same linear measurement continuum of physical fatigue. The sample distribution a represents the total score estimates for the physical fatigue scale plotted on a continuum of physical fatigue severity ranging from left (low severity) to right (high severity). The five-category response scale leads to four thresholds for each item. Therefore, the item threshold distribution b represents each of the four thresholds estimates for each item, plotted on the lower end of the same measurement continuum of physical fatigue. A threshold reflects the location on the measurement continuum where two adjacent response categories are equally likely to be endorsed. Targeting is assessed by examining the relative range and coverage of the sample distribution by the available item thresholds. The lower part of the figure c depicts the 15 items of the physical fatigue scale in the y-axis in order of increasing difficulty from top to bottom. The x-axis represents the most probable of the five response categories in the different coloured blocks across the range of the physical fatigue continuum. RMT expects the ordering of the response categories to reflect the intended severity i.e., from none of the time to all the time