1. Sample Design of the National Health Interview Survey (NHIS) Linda Tompkins Data Users Conference July 12, 2006 Centers for Disease Control and Prevention National Center for Health Statistics

2. Outline Overview of the NHIS Design features Sample weights and estimation Summary

3. Overview of the National Health Interview Survey (NHIS) Nationally representative sample of the civilian, noninstitutionalized population of the United States Face-to-face personal interviews Continuously conducted since 1957 Topics cover a broad range of health issues and provide information on both acute and chronic conditions

4. NHIS sample design Has a “complex” design (multistage design includes clustering, stratification) 428 primary sampling units (PSUs) drawn from approximately 1800 geographic areas covering the 50 states and the District of Columbia Sampling geographic areas helps to control survey costs

5. NHIS sample design (cont.) Approximately 40,000 households containing almost 100,000 persons are selected from Census-defined tracts and block groups Currently oversampling Blacks, Hispanics, Asians, and elderly minorities in these groups Detailed health information collected from one sample adult and one sample child per household

6. 2006 NHIS poststrata HispanicNonhispanic Black Nonhispanic AsianOther Age GroupsAge Groups Under 1 yearUnder 1 year Under 5 years Under 1 year 1 ‑ 4 years1 ‑ 4 years 5-17 years1 ‑ 4 years 5 ‑ 9 years5 ‑ 9 years 18-24 years5 ‑ 9 years 10 ‑ 14 years10 ‑ 14 years 25-44 years10 ‑ 14 years 15 ‑ 17 years15 ‑ 17 years 45-64 years15 ‑ 17 years 18 ‑ 19 years18 ‑ 19 years 65 + years 18 ‑ 19 years 20 ‑ 24 years20 ‑ 24 years20 ‑ 24 years 25 ‑ 29 years25 ‑ 29 years25 ‑ 29 years 30 ‑ 34 years30 ‑ 34 years30 ‑ 34 years 35 ‑ 44 years35 ‑ 44 years35 ‑ 44 years 45 ‑ 49 years45 ‑ 49 years45 ‑ 49 years 50 ‑ 54 years50 ‑ 54 years50 ‑ 54 years 55 ‑ 64 years55 ‑ 64 years55 ‑ 64 years 65 + years65 ‑ 74 years65 ‑ 74 years 75 + years 75 + years

7. NHIS sample weights Data from all probability sample surveys have weights, resulting from sample selection probabilities Weights vary, due primarily to oversampling Blacks, Hispanics, Asians to provide improved estimates

8. NHIS sample weights (cont.) Weights composed of four components: –The reciprocal of the probability of selection –A household nonresponse adjustment –A first-stage ratio adjustment –A second stage ratio (or poststratification) adjustment to the U.S. population by age, sex, and race ethnicity

9. NHIS variance estimation Must take into account the complex nature of the sample design Taylor series linearization technique Appropriate complex variance estimation software (e.g., SAS survey procedures, SUDAAN, SPSS survey module)

10. NHIS variance estimation (cont.) Estimates based on a sample contain sampling variability Estimates based on a simple random sample will most likely underestimate the actual sampling variance and produce incorrect estimates (such as totals, ratios, rates) Degrees of freedom for hypothesis tests would be too large

11. Summary The NHIS, a household, face-to-face survey, has been conducted continuously since 1957 to provide estimates information on both acute and chronic conditions in the U.S. population Sample design is complex, where the sample is geographically clustered to reduce personal interview costs

12. Summary (cont.) Sampling weights should be used when computing estimates using NHIS data Complex design variance estimation software should be used Redesign of the NHIS for 2015-2024 is now underway

13. NHIS sample design references Design and Estimation for the National Health Interview Survey, 1995-2004. Series 2, No. 130 (2000) National Health Interview Survey: Research for the 1995-2004 Design. Series 2, No. 126 (1999)

14. For further information: www.cdc.gov/nchs Linda Tompkins Phone: (301) 458-4533 Email: lit3@cdc.gov Centers for Disease Control and Prevention National Center for Health Statistics