After publication of the Flexner report in 1910, admission and graduation requirements for U.S. medical schools became very rigorous, and adherence to the scientific method in patient care and research was emphasized. Many medical schools were closed and most that met the new scientific criteria were university-based. Post-graduate internships were required for all practitioners in the 1920s, and by the 1930s, all sub-specialists were required to complete residency training in their field.