Case-mix adjustments for patient reported experience and outcome measures in primary care: an empirical approach to identify patient characteristics as case-mix adjusters based on a secondary analysis of an international survey among patients and their general practitioners in 34 countries.

Our approach provides guidance to decide whether or not patient characteristics should be considered as case-mix adjusters. The criterion of a difference between variances of 0.25*variance works well for continuous PREMs and PROMs, but seems to be too strict for binary PREMs and PROMs. Without additional information, it is not possible to decide whether important slope variation is the result of either differences in performance between general practices or countries, or cultural differences.