1. American mode
The “4+4” model established by United States (US) is well known: medical students need to complete 4 years of general university education and receive a bachelor’s degree before entering medical school. After admission to medical school, medical students undergo four years of medical education, culminating in an M.D. degree [23]. Earning an M.D. is a prerequisite for admission to practice medicine. Medical students enter the residency training after completing the medical school [24]. They register as general practitioners after the first phase of training, and as specialists after the second phase of training in related specialties. Many medical schools in the US also promote the M.D. / Ph.D. program, which is to add the Ph.D. training to M.D.[25]. It usually requires 4-5 years of research training with the goal of developing leaders in the field of medical research.