When we use instruments to evaluate treatment effects, they must be able to measure differences between groups, if differences do in fact exist. Randomization should ensure that participants in experimental and control intervention groups begin studies with the same status on whatever concept or construct the PRO is designed to measure. PROs must be able to detect what is important to patients and distinguish among participants who remain the same, improve, or deteriorate over the course of the trial. This is sometimes referred to as responsiveness, or sensitivity to change.
An instrument with a poor ability to measure change can result in false-negative results in which the experimental intervention improves how patients feel, yet the instrument fails to detect the improvement. This problem may be particularly salient for generic questionnaires that have the advantage of covering all relevant areas of HRQOL, but the disadvantage of covering each area superficially. In studies that show no difference in PROs between experimental and control intervention, lack of instrument responsiveness is one possible reason.