A propensity score is a statistical tool used to estimate the effect of a treatment or intervention in observational studies. It represents the probability that a participant would receive a particular treatment based on their observed characteristics. By matching or grouping individuals with similar propensity scores, researchers can reduce bias and make more accurate comparisons between treated and untreated groups. This method is particularly useful in fields like epidemiology and social sciences, where random assignment to treatment groups is often not feasible. By controlling for confounding variables, propensity scores help ensure that the groups being compared are as similar as possible, leading to more reliable conclusions about the treatment's effectiveness.