In the US, accreditation and certification of residency training are functions of separate public sector agencies. Accrediting decisions are made directly by 26 Residency Review Committees, which represent the primary medical specialties and function under the authority of the Accreditation Council for Graduate Medical Education. The accrediting bodies may consider only educational issues and are prohibited by the government from controlling physician supply. Only the programme, not the institution in which it is conducted, is accredited. The US residency is a structured educational programme that is expected to provide comparable experience to all enrolled residents. Length of training may vary from two to six years depending on the specialty. Additional training may be obtained in subspecialty programmes, which are subsets of the primary specialty residencies and are also reviewed for accreditation. These have increased in significant number in recent years as subspecialisation has proliferated in the US.