Psychological measures are important because they can influence the expression of pain and physical function in patients with arthritis. A number of instruments are now available that measure psychological distress and how we as individuals manage stress. These instruments have undergone extensive validation, although more work is required to evaluate the performance of these instruments measuring change over time. One way to interpret psychological measures and to evaluate how they change over time is to use normative comparisons that are conditional on time and other relevant covariates, using statistical methods such as quantile regression. Such methods have been used to interpret the developmental, educational, and physical growth of children. We can use similar methods to interpret observational studies and to guide decisions within the context of clinical practice.