Objective To examine the proportion of healthcare visits are delivered by nurse practitioners and physician assistants versus physicians and how this has changed over time and by clinical setting, diagnosis, and patient demographics. Design Cross-sectional time series study. Setting National data from the traditional Medicare insurance program in the USA. Participants Of people using Medicare (ie, those older than 65 years, permanently disabled, and people with end stage renal disease), a 20% random sample was taken. Main outcome measures The proportion of physician, nurse practitioner, and ph...