# Presentation: 702385

## 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**

## Outline

- Overview of the NHIS
- Design features
- Sample weights and estimation
- Summary

## 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

**Notes:**
Interviews conducted by Computer Assisted Personal Interview (CAPI) using Blaise software
Topics include:  in addition to basic demographic information, limitation of activities, injuries, access to health insurance and health care, injuries, and health behaviors (such as tobacco and alcohol usage, physical activities).

## 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

**Notes:**
1.	PSUs are individual counties or contiguous groups of counties and vary in size from a few hundred to several million.  The NHIS is not designed to produce reliable state-level estimates.
Simple random sampling would be costly (sampling individual members of the population), would not assure represenation of special subgroups), and virtually impossible, since not complete, up-to-date listing of individual members of the population exists.

The NHIS sample
Is it "random"?
Cost / administrative considerations that influence the sample design

3.  Sampling geographic areas helps to control survey costs
Travel, administration costs decreased for personal visit interviewing, relative to a simple random sample 
Sample is designed to give improved estimates for several groups (Black persons, Hispanic persons)

## 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

## 2006 NHIS poststrata

**Hispanic****	****Nonhispanic Black     Nonhispanic Asian****	****Other**
- _Age Groups_	_Age Groups_	      _Age Groups_		_Age Groups_
- Under 1 year	Under 1 year	      Under 5 years	 	Under 1 year
- 1‐4 years	1‐4 years	      5-17 years		1‐4 years
- 5‐9 years	5‐9 years	     18-24 years		5‐9 years
- 10‐14 years	10‐14 years	     25-44 years		10‐14 years
- 15‐17 years	15‐17 years	     45-64 years		15‐17 years
- 18‐19 years	18‐19 years	     65 + years 		18‐19 years
- 20‐24 years	20‐24 years				20‐24 years
- 25‐29 years	25‐29 years				25‐29 years
- 30‐34 years	30‐34 years				30‐34 years
- 35‐44 years	35‐44 years				35‐44 years
- 45‐49 years	45‐49 years				45‐49 years
- 50‐54 years	50‐54 years				50‐54 years
- 55‐64 years	55‐64 years				55‐64 years
- 65 + years	65‐74 years				65‐74 years
-           75 + years 			 	75 + years

**Notes:**
The preliminary poststrata for the 2006 NHIS Person File include additional poststrata for Non-Hispanic Asians.  The following 50 groups are used separately for males and females:

## 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

## 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

**Notes:**
US population figures are obtained from the Census Bureau

## 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)

**Notes:**
Complex:  unequal selection probabilities, clustering of households and respondents
SUDAAN callable available (SAS-like procs) 
R is free and downloadable

## 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

## 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

## 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

**Notes:**
Unweighted estimates of totals would be too small
Unweighted estimates of rates and other ratios could be distorted; unweighted sample proportions of certain groups (e.g., Black persons) would be different from the corresponding  U.S. population proportions

## 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)

**Notes:**
Both documents are available at the NCHS website

## 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**

**Notes:**
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)