For 1986 to 1996, the NHIS Field Representative's Manuals generally defined an HMO as a "health care plan that delivers comprehensive, coordinated medical services to enrolled members on a prepaid basis." (Prior to 1986, the Manuals did not define an HMO.) The Manuals further described listed three types of HMOs.

The Manuals noted that "an HMO differs from other health insurance because it directly provides its members with most or all of their health care, while traditional health insurers simply process the claims. An HMO assumes responsibility for providing the treatment as well as paying the bills."

For 1997 forward, the Manuals defined HMOs as "health delivery systems that offer comprehensive health coverage for hospital and physician services for a prepaid, fixed fee."

For 1976 to 1992, private health insurance excluded single service plans that paid for only one type of service (such as nursing home care, accidents, or dental care). For 1976 to 1989, plans that paid extra cash while hospitalized counted as private insurance; such "extra cash plans" are not mentioned in the Manuals for 1992 and 1996.

For 1993 to 1996, single service plans were considered to be health insurance; however, only respondents who indicated their plan was a general purpose plan were asked the question for HIPTYPER. For 1997 and 1998, the definition of private insurance excluded single service plans. For 1999 forward, a separate category indicating single service plans was added to the survey, to better distinguish single service plans from comprehensive private health insurance.

For 1976 to 1989, the general definition of private insurance in the Field Representative's Manuals referred to plans "specifically designed to pay all or part of the hospital, doctor, surgeon, or other medical expenses of the insured individual" (including dentist expenses for 1989 and 1986). The Manuals further stated, "The plan, in order to be considered as insurance, must be a formal one with defined membership and benefits. Include Health Maintenance Organizations (HMOs) and Individual Practice Associations (IPAs), such as Kaiser, Group Health, etc."

For 1990 forward, the Manuals defined private health insurance to be any type of health insurance, including Health Maintenance Organizations (HMOs), other than the public programs of Medicare, Medicaid, Military health care/VA, CHAMPUS/TRICARE/CHAMP-VA, Indian Health Service, state-sponsored health plans, CHIP (the Children's Health Insurance Plan, beginning in 1999), and health insurance from other government programs.

1997 forward Read If Necessary: Health Maintenance Organizations, or HMOs, and Individual Practice Associations, or IPAs, are plans whose members are required to use only those doctors who work for or in association with the plan. Sometimes members may choose to go to doctors not associated with the plan, but usually at greater cost to the member. Generally, members do not have to submit claims for costs of medical care services.

• Is {plan name} an HMO (Health Maintenance Organization), an IPA (Individual Practice Association), or is it some other kind of plan?

• Is (plan name) an HMO (Health Maintenance Organization) or IPA (Individual Practice Association), or is it some other kind of plan?

• Is this (name) plan a Health Maintenance Organization or HMO?

Persons with three or more plans have a "yes" response to HIPRIVGT2 (available for 2004 forward). Analysis by IHIS staff indicates that a very small number of persons (less than 0.2 percent of the total number of individuals with private health insurance per year for 2004 to 2009) had three or more private insurance plans.