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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. Approach and Application. Dual-Frame Approach for a Survey That Screens for a Rare Population
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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
Approach and Application • Dual-Frame Approach for a Survey That Screens for a Rare Population • Potential Application to the National Immunization Survey (NIS)
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.
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
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.
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.
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.
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
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
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
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
Number of Completes • St RDD Completes Experian Completes Total • 86 40 126 • 75 35 110 • 69 39 108 • 65 22 87 • 70 35 105
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.