From the moment of birth to the signing of the death certificate, medical records are maintained on almost every individual in the United States (and many other countries). Increasing quantities of data are abstracted from written records, or entered directly at a workstation, and submitted by providers of healthcare to payer and regulatory organizations. Providers include physician offices, clinics, and hospitals, payers include managed care corporations and insurance companies, and regulatory organizations include state and federal government. Trends are towards making the flow of data easier, more comprehensive, and multi-faceted: through EDI (electronic data interchange), CHINs (Community Health Information Networks), and a seemingly ever more intrusive, detailed, and specific involvement by payors in the handling of care and compensation by and for providers.

These so-called clinical and financial administrative health care data are routinely massive. The Health Care Financing Administration's annual MEDPAR data base contains around 14 million discharge abstracts of every medicare-funded acute-care hospital stay. Individual state's administrative data of hospital discharges may include several million records annually. Data are collected in certain standard formats, including uniform billing (UB82, now UB92) for administrative data on hospital stays, and HEDIS (1.0, 2.0, 3.0) on patients in managed care. The more detailed data is often proprietary: for example HEDIS data is often proprietary to the specific payer organization, and includes data only on the organization's enrollees. More ambitious data collecting is underway in selected locations, through the systematic abstraction of supplementary clinical measures of patient health from medical records, through recording of additional patient characteristics, or through recording of more detailed financial information.

In principal the entire medical record is available. A written version might be available online in digital form as an image. Record linkage, for example between members of a family (mother and child), or through the use of unique patient identifiers across multiple episodes of treatment, or from administrative data to the registry of vital statistics (death certificates) and to cancer registries, provides important additional information. However, the availability of such information is, again, restricted.

A traditional uses of health data is in public health assessment and the evaluation of clinical efficacy of particular treatments and interventions.