These sources are usually provided for defined populations (i.e., cities, states, political entities). Morbidity data have usually been hospital and clinic data, which means the populations of reference may not be easy to define.

Changes in health care delivery will have an impact on hospital and clinic data. In the U.S., the trend has been to hospitalize only the most sickest, and then for the shortest length of time. For example, “length of stay” may no longer be truly reflective of “severity of illness.”