CASE STUDY: NATIONAL SURVEY OF FAMILY GROWTH

Karen E. DavisNational Center for Health Statistics

Coordinating Center for Health Information and Service

NCHS Data Users Conference, Washington, D.C. July 12, 2006

NCHS SURVEYS• Records based

– Data from vital and medical records– e.g. NDI, VSCP

• Population based– Data from personal interviews– e.g. NHIS, NSFG, NIS, SLAITS,NHANES

• Establishment based– Data from hospital records/facility interviews – e.g. NAMCS, NHAMCS, NHDS, NNHS

3

PRESENTATION OUTLINE

• The NSFG Cycle 6 Sample• SRS versus complex?• Estimates from NSFG Data• Cycle 7 Sample

4

NSFG Background

• NSFG data are a source of information on childbearing, pregnancy, and related events

• Collects statistics on family formation, childbearing, marriage, divorce, and cohabitation

• Conducted periodically: 1973, 1976, 1982, 1988, 1995, 2002-2003

5

NSFG Cycle 6

• Previous cycles were nationally representative area samples of the civilian noninstitutionalized population of women

• Cycle 6 surveys both men and women 15-44 years of age

• Main data collection Mar. 2002–Feb. 2003

6

Key Analytical Goals of Cycle 6

• Compare key statistics within Cycle 6 by race/ethnicity and age

• Compare key statistics between survey cycles

7

Key Statistics of Cycle 6

• Proportion of teenagers who have ever had sexual intercourse;

• Proportion of teenagers who used a condom at their most recent intercourse;

• Proportion of each 5-year age group who are currently using the oral contraceptive pill; and

• Proportion of childless women by age who have impaired fecundity.

8

Cycle 6 Sample Design

• 121 PSU stratified multistage cluster sample of households

• Primary selection: blocks– Select 1,414 segments– Select 40 households per segment

• 55,000 selected housing units

9

Cycle 6 Distribution by Region

10

Region Number of Segments Number of HUs

Northeast 341 13,420

Southeast 336 13,145

West 399 15,675

Midwest 324 12,760

Total 1400 55,000

Source: ISR

Cycle 6 Sample Design Features

• 12,571 completed interviews

– Females and Males 15-44

– Higher rates for African-Americans, Hispanics and 15-19 year-olds

11

Cycle 6 Analytical Subdomains

12

Age

Females Males

Black Hispanic Other Black Hispanic Other

15-19

20-24

25-44

Cycle 6 Sample Design Features

• Within Household Selection

– Select one eligible person at random

– Implement selection in Blaise software– Age-gender-race/ethnicity selection cells– Vary sampling rates across cells

13

Cycle 6 Sample Design

Why not use Simple Random Sampling?

14

Simple Random Sampling

• All frame elements have an equal chance of selection

• All combinations of frame elements (of the given sample size) have an equal chance of selection

15

Simple Random Sampling

• Costly data collection – sampling individual members of the population

• Does not assure representation of special sub-groups of interest

• No complete, up-to-date listing of individual members of the population

16

Use of complex designs

• Cluster sampling is a low-cost device for fixing the probability of including each member of the population in the sample

• Complete listings of clusters (e.g. counties) are readily available

• Special subgroups are sampled at higher rates to assure sample size

17

Use of complex designs

• Can reduce sampling error over simple random sampling depending on allocation of strata

• Can reduce travel or other data collection costs• Unequal selection probabilities can increase the

sample size of rare units

18

Cycle 6 Sample Design

• Complex design is essential for NSFG targets:

– Females and Males 15-44

– Higher rates for African-Americans, Hispanics and 15-19 year-olds

19

Cycle 6 Responsive DesignMore timely data on field costs and response rates • ISR developed SurveyTrak system for daily info on

hours of effort required to obtain interviews• Field administrative data used for statistical

modeling and statistical process control analysis• Prevent cost over-runs and manage fieldwork given

limited budget• Implement double sample in final month

20

NSFG Weighting

• The product of four factors is used to create a single weight for each case:

• Factor 1: Base sampling weights – Inverse of selection probabilities

• Factor 2: First-stage ratio adjustment– Counteracts sampling variation across primary

sampling units within a stratum

21

NSFG Weighting

• Factor 3: Nonresponse adjustment– Includes eligibility, noncontact, and refusal

adjustments

• Factor 4: Poststratification adjustment– Uses external population totals for ratio

adjustments by age, gender, and race/ethnicity

22

NSFG Imputation

• Logical Imputation– Deduce missing answer from answers to other

questions

• Multivariate sequential regression– Model based on nonmissing predictors

23

NSFG Variance Estimation

• Should reflect complex design including unequal selection probabilities and clustering of respondents

• A variance estimate based on a simple random sample will likely underestimate the actual sampling variance

24

NSFG Variance Estimation

• Taylor Series or pseudo-replication

• Complex variance estimation software (e.g. SUDAAN, WesVarPC, SAS)

25

NSFG Design Effects

• Provide a summary measure of the combined effects of stratification, clustering, and unequal weighting on the variance of a survey estimate.

• Design Effects are generally larger for subgroups that are oversampled because of the sample design.

26

NSFG Cycle 7

Continuous interviewing• 4400 male and female respondents per year• Content similar to Cycle 6• Collect data more frequentlyResponsive design• Reduce cost per case

27

NSFG Cycle 7 Sample Design

• Use the same 8 large Metro areas each year, plus 25 new PSUs each year

– Year 1: 33 PSUs (8 +25) (2006-2007)– Year 2: 58 PSUs (8 +50) (2008)– Year 3: 83 PSUs (8 + 75) (2009)– Year 4: 108 PSUs (8 + 100) (2010)

28

Sample Size Yields for Cycle 7

29

Cycle 6 Cycle 7

(2002)Jun 2006-Dec 2007

Jun 2006- Dec 2010

Total 12,571 6,600 19,800

15-19 2,271 1,200 3,100

Male 4,928 2,970 8,900

Female 7,643 3,630 10,900

Hispanic 2,712 1,313 3,940

Black 2,460 1,340 4,020

Source: ISR

References for the NSFG Design: NCHS Publications

National Survey of Family Growth, Cycle 6: Sample Design, Weighting, Imputation and Variance Estimation. Series 2, No. 142 (June, 2006)

Plan and Operation of Cycle 6 of the National Survey of Family Growth. Series 1, No. 42 (August, 2005)

Both are available at NCHS website.

30

NCHS website:www.cdc.gov/nchs/

• Contact: Karen E. Davis Email: kedavis@cdc.gov

31