A value used in the interpretation of the results of an experimental design in which the allocation of treatments to experimental units was not made at random. A common example concerns a hospital patient, where the range of feasible treatments may be limited by aspects of the patient’s condition. The propensity score is defined as the conditional probability of the patient receiving a particular treatment given the values of relevant characteristics (e.g. age, sex) of the patient. The score is typically estimated using a logistic regression model.