Overview of the consensus-based operational definition of contextual factors. The three contextual factor types describe different ways that contextual factors can influence the results of a trial. Brief descriptions of each type are shown in the figure. All three types are described in detail in the material below. In short, EM-CFs modify the treatment effect (i.e. some patient subgroups experience greater or less effect from a treatment compared to other subgroups). OI-CFs are prognostic factors (sometimes called risk factors), i.e. factors predicting the course of a patient’s condition and may confound the results of trials that are not randomized. MA-CFs influence the performance of outcome measurement instruments (such as reliability, validity, responsiveness, etc.). To guide which specific factors could be considered contextual factors, the factors must fit within one of the three classification categories, i.e. either personal-, disease-related, or environmental factors. The contextual factor types are not mutually exclusive, so some specific factors, e.g. sex, may both be an EM-CF, OI-CF, and MA-CF. CFs, Contextual Factors. Figure published in Nielsen et al. Semin Arthritis Rheum. 2021 Jun;51(3):601-606.