A Dual-Frame Design for an RDD Survey That Screens for a Rare Population

1. A Dual-Frame Design for an RDD Survey That Screens for a Rare Population K.P. Srinath, Abt Associates Inc. Michael P. Battaglia, Abt Associates Inc. Meena Khare, NCHS, CDC

2. Approach and Application • Dual-Frame Approach for a Survey That Screens for a Rare Population • Potential Application to the National Immunization Survey (NIS)

3. RDD Surveys • Many large surveys use list-assisted random-digit-dialing (RDD) samples. • Some surveys focus on a specific target population. • Example: NIS- children between 19 and 35 months of age.

4. Screening Households for Eligibility • Large pool of interviewers • Considerable time and effort • Substantial cost • Adds to nonresponse because of screening attempts • Nonresponse correlated with eligibility criteria

5. Dual-Frame Approach • Frame A: • RDD frame containing entire population of telephone numbers • Frame A offers complete coverage of the target population. • Frame B: • List frame: Telephone numbers of households in targeted population. • Frame B is a subset of Frame A and is incomplete. • We select simple random samples from each frame.

6. Notation

7. Notation, cont’d.

8. Notation, cont’d

9. Notation, cont’d.

10. Notation, cont'd.

11. Notation,cont’d.

12. Estimator

13. Estimator, cont’d.

14. Estimator, cont’d.

15. Variance

16. Minimizing Variance

17. Optimum Values

18. Total Screening Sample Size

19. Allocation for a given “p”

20. National Immunization Survey • Large ongoing RDD survey, conducted quarterly by the CDC. • Measures vaccination coverage rates among children aged 19-35 months. • 78 geographic strata • Quarterly list-assisted RDD samples • Only 3.4% of the households contain age-eligible children.

21. Dual Frame • RDD frame offers coverage of entire population of residential telephone numbers except those in “0” banks. • Commercial lists vary in quality of information and coverage of eligible population. • Experian New Babies List is available for all 50 states and D.C.

22. Dual-Frame Experiment • Identified 5 urban strata for a test, conducted in Q4/2003. • Selected random sample of telephone numbers from the New Babies List. • Selected the RDD sample. • Matched the RDD sample with the Experian New Babies List sampling frame

23. Results from Q4/2003 • St RDD Numbers(M)Experian(Mb) Alpha(%) • 608,400 986 0.162 • 1,604,500 3,875 0.241 • 699,300 2,356 0.337 • 937,000 4,479 0.478 • 5 457,200 1,739 0.380

24. Observed Eligibility Rates • St. Overall (e) Experian (eb) p • sqrt(ea/eb) • 1.28 23.8 0.229 • 1.17 21.6 0.228 • 1.50 20.6 0.264 • 1.73 28.7 0.236 • 1.13 28.8 0.189

25. Screening Sample Size • S RDD Sample Dual Frame m mb Diff. • 9,844 6, 888 6720 168 2,956 • 9,402 6,411 6,411 162 2,991 • 7,200 4,790 4,600 190 2,600 • 5,029 3,835 3,758 77 1,194 • 9,292 6,317 6,195 122 2,975

26. Number of Completes • St RDD Completes Experian Completes Total • 86 40 126 • 75 35 110 • 69 39 108 • 65 22 87 • 70 35 105

27. Conclusions • Dual-frame designs can offer savings during data collection. • Experian New Babies List performed reasonably well. • Need estimates of sample design parameters to plan allocation of the sample to the two frames. • Need to balance saving in screening cost and possible increase in variance.

28. Abt Associates Inc.