5 May 2020 - Evidence obtained from clinical practice settings that compares alternative treatments is an important source of information about populations and end points for which randomised clinical trials are unavailable or infeasible.

Unlike clinical trials, which strive to ensure patient characteristics are comparable across treatment groups through randomisation, observational studies must attempt to adjust for differences (i.e, confounding). 

This is frequently addressed with a propensity score (PS) that summarises differences in patient characteristics between treatment groups. The PS is the probability that each individual will be assigned to receive the treatment of interest given their measured covariates.

Read JAMA Guide to Statistics and Methods