The program became law in 1965. Medicaid is administered by state agencies and is jointly funded by the federal, state, and, sometimes, local governments. Eligibility requirements for this means-tested program vary across states. Most recipients of public welfare income programs such as TANF (Temporary Assistance to Needy Families) and SSI (Supplemental Security Income) are eligible for Medicaid coverage. In some states, other persons qualify, such as needy unemployed persons who have children and who are not receiving cash assistance, and medically needy persons whose income and assets are too low to cover their medical costs. Many Medicaid recipients are inmates of medical institutions, such as low-income elderly persons in nursing homes. Such institutionalized persons are not included in the NHIS sample, which covers the civilian non-institutionalized population of the United States.

These linked broad categories of insurance that were studied in the survey to recognizable public health insurance program names. For example, in 1999, the California Medicaid program was called the "Medi-Cal" or "Medi-Cal Managed Care" or "The Two-Plan Model."

Categories included:

• Private health insurance (2004 forward)

• Private health insurance plan from employer or workplace (1997-2003)

• Private health insurance plan purchased directly (1997-2003)

• Private health insurance plan through a State or local government program or community program (1998-2003)

• Medicare (1997 forward)

• Medi-Gap (1997 forward)

• Medicaid (1997 forward)

• CHIP (Children's Health Insurance Program) (1999 forward)

• Military Health Care/VA (1997-2003)

• Military Health Care (CHAMPUS/TRICARE/CHAMP-VA) (1997-2018)

• Military related health care: TRICARE (CHAMPUS)/VA health care/CHAMP-VA (2019 forward)

• Indian Health Service (1997 forward)

• State-sponsored health plan (1997 forward)

• Other government program (1997 forward)

• Single Service Plan (e.g., dental, vision, prescriptions) (1999-2018)

• No coverage of any type (2000 forward)

Respondents were consistently instructed to exclude private plans that "only provide extra cash while hospitalized." In 1997-1998 respondents were also instructed to "EXCLUDE private plans that ... pay for only one type of service (nursing home care, accidents, or dental care)." Between 1999 and 2018, "Single Service Plan" was added as a possible response, and, consequently, the instructions were changed to read, "INCLUDE those [private plans] that pay for only one type of service (nursing home care, accidents, or dental care)." Beginning in 2019, respondents were asked three separate questions to determine if they had single service plans for dental services, vision services, and/or prescriptions.

In follow-up questions, interviewers recorded the names of up to four private health insurance plans. If the person was reported as covered by CHIP (beginning in 2000), by a state-sponsored health plan, or by another public program (other than Medicaid) that paid for health care, the interviewer recorded the name of that plan. The placement and wording of these questions about the names of specific government health care plans varied across years.

For persons initially reported as not having health care coverage of any kind, interviewers asked whether the person had Medicare coverage, Medicaid coverage, coverage via a Medicaid program or non-Medicaid state-sponsored health insurance program with the appropriate name for the state, CHAMPUS or CHAMPVA coverage, or any private insurance. For those who acknowledged any such coverage, the interviewer repeated the series of questions mentioned above to determine the specific type of coverage.

This might be shown, for example, by a mismatch between the verbatim name of an insurance plan and the type of insurance coverage the person selected from the categories on the card.

In addition to HIMCAIDE, IPUMS NHIS contains the following recoded insurance variables: HICHIPE (Covered by Children's Health Insurance Program: Recode); HIPRIVATEE (Covered by private health insurance: Recode); HIMILITE (Covered by military health insurance: Recode); HIHSE (Covered by Indian Health Service: Recode); HIMCAREE (Covered by Medicare: Recode); HISTATEE (Covered by other state-sponsored health plan: Recode); HIOTHGOVE (Covered by other government program: Recode); and HINOTCOVE (No coverage of any type: Recode).