Instrument sensitivity and disease severity. Physical Function, on the horizontal scale, is mapped against sensitivity (reliability) on the vertical scale. A better scale has a greater breadth of Physical Function ability. The lower the curve, the greater the sensitivity. Across the overall range, the PROMIS PF 20 is most sensitive, maintaining a reliability of 0.95 over more than four SD. The Item-Improved HAQ is superior to the Item-Improved PF-10 in populations with function more than 1.2 SD from the mean (for example, Rheumatoid Arthritis) whereas the opposite is true in populations with functional abilities better than this (for example, general population-based). HAQ: health assessment questionnaire; Item-Improved: Changes to items intended to improve their performance by altering parameters such as time frame and response options while retaining the same item stem; PF, physical function; PROMIS: Patient-Reported Outcomes Measurement Information System; SD, standard deviation.