The National Children's Study 2014 An Assessment (2014) / Chapter Skim
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3 Sample Design
3 Sample Design
Pages 45-70
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From page 45... ...
It ends by recommending next steps. Taken together, the chapter addresses statistical issues surrounding the following key items the panel was asked to consider: the national probability sample's overall sample size and design; the use of hospitals and birthing centers as the primary sampling unit; the use of health care providers to sample and recruit prospective participants; the relative size of the prenatal and birth strata in the probability sample; and the optimal use of sibling births.
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From page 46... ...
In practice, each PSU will possibly have different overlapping 4-year periods, the "birth window," during which births are enrolled in the study, because the start of the sample enrollment will be rolled out over time.2 While the broad outline of this current sample design was provided to the panel, many crucial details of the sample design had not yet been resolved as this report was being written.3 The probability sample would use birthing hospitals and birthing centers as the PSUs and prenatal care providers whose patients deliver at the selected hospitals and birthing centers as the SSUs.4 The list of providers associated with selected hospitals (the SSU frame) would be split into two strata: the details of this split were not provided to the panel.
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From page 47... ...
For the prenatal stratum, there will be sampling of eligible women at their "first prenatal visit" to a sampled provider. For the birth stratum, there will be sampling of eligible women who have a live birth at the selected hospitals and birthing centers.
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From page 48... ...
Typical sampling plans describe a study's objectives and constraints and the steps proposed to operationalize and realize those objectives. Examples of surveys with readily-available sampling plans include the National Survey on Drug Use and Health,7 the National Survey of Child and Adolescent WellBeing,8 the Current Population Survey,9 the National Longitudinal Study of Youth 1997,10 and the National Postsecondary Student Aid Study.11 These reports provide more detail than the NCS Program Office can be expected to 5 NCS wrote that the "specific rollout plan is still under development" (NICHD, 2013j, p.
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From page 49... ...
In the case of the NCS design, the sampling protocol for subsequent siblings should be provided in detail. The plan should include a rigorous determination of the overall inclusion probabilities, expected yield rates, response rates, and retention rates at each stage (hospital, provider, birth)
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From page 50... ...
Target Population The target population for the proposed design is all live births in the United States during a 4-year time period, referred to as the inference period in this document. Although they are part of the NCS target population, two types of births are excluded from the NCS sample frame: births to women who do not deliver in birthing hospitals or birthing centers and births to women at hospitals in which there are too few births to be included in the sampling frame.
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From page 51... ...
The design document does not state whether the same size threshold criterion would also be used for birthing centers, or even whether birthing centers are included in the database. If birthing centers are included on the same list frame with hospitals and a single size threshold is used, they are unlikely to be adequately represented in a probability proportional to size sample because they tend to have a much smaller number of births than birthing hospitals.
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From page 52... ...
We expect that women recruited during these sampling times who end up delivering earlier or later than expected, and who therefore do not deliver during the birth window, would later be considered ineligible for the NCS. CONCLUSION 3-1: The panel endorses the proposed target population of all births in the United States during a specified time period consisting of 4 full calendar years, as well as the proposed sample exclusions from this target population.
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From page 53... ...
CONCLUSION 3-2: Because of the lack of explicit hypotheses in the study design, it is not possible for the panel to judge whether the proposed sample size is justified on the basis of the study's objectives. Equal Probability Sample The stated goal of the NCS is to have an equal probability of selection sample to the extent possible.17 The intent is to select each newborn, either 16 this case, the stated goal is to identify an odds ratio of 2 or greater with a power of 0.8 and In a two-tailed significance level of 0.05.
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From page 54... ...
judged that the lack of any oversampling of population subgroups is justified because the planned sample size of 100,000 would provide sample sizes for major demographic subgroups that are large enough to provide adequate statistical power across a number of subgroups of interest for research on health disparities. The current panel reconsidered this issue, as well as whether oversampling might be needed to adjust for analytically interesting population subgroups, such as low-birth-weight babies to families of low socioeconomic status, who may have higher expected nonresponse and attrition rates.
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From page 55... ...
Most national longitudinal studies, such as the Early Childhood Longitudinal Study -- Birth Cohort Study and the Fragile Families and Child Well-being Study,19 have suffered disproportionate attrition among socially disadvantaged groups. A strategy to counterbalance the analytic impacts of disproportionate attrition is to oversample disadvantaged groups at the beginning of a study relative to their likely attrition patterns through the middle or end of the follow-up period.
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From page 56... ...
The presence of sufficient variability in socioeconomic status and geography within race and ethnic groups is fundamental to answering key questions about health disparities. The brief PSU design summary the panel received (NICHD, 2013i)
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From page 57... ...
are available for various sampling units, whether counties, hospitals, or provider locations. The currently proposed plan to split the provider sample frame to enable half of the sample of births to enter the sample from prenatal providers and half from hospitals is an atypical form of stratification.
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From page 58... ...
Hospitals were not used as sampling units. Hospital-based recruitment was tested in the Vanguard Study by targeting three hospitals selected in each of three geographic PSUs, asking them to collect birth specimens for all women in order to have the information for women recruited after birth.
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From page 59... ...
As noted above, a previous design, which was tested in three Vanguard Study locations, used geography to define the first-stage sample, with prenatal care providers selected from the sampled geographic areas, followed by recruitment of women from the providers. One of the challenges with this plan was the number of hospitals that would need to be enlisted to collect birth specimens for women already enrolled in the study.
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From page 60... ...
and based on Vanguard Study pilot testing -- namely, county-based primary sampling units with hospitals as secondary sampling units and providers as third stage sampling units. Because the current plan calls for hospitals to be selected with probability proportional to size, it is important that a good measure of size be available for each hospital on the frame.
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From page 61... ...
Even though many procedures that are being proposed for the Main Study have been tested in the Vanguard Study, the panel was not provided with detailed information about all approaches that were tested and how well they worked. Some detail and discussion of the NCS provider-based sampling experience has been published by former Vanguard Study principal investigators (see, e.g., Belanger et al., 2013)
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From page 62... ...
The Vanguard Study conducted a limited hospital-based sampling approach (proposed for the birth stratum)
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From page 63... ...
and the associated cumulative nonparticipation rates appear to be high.28 Aside from nonparticipation of hospitals and providers,29 which will be addressed using sample substitution and perhaps not included in the denominator of the response rate, it appears that provider-based sampling of women in the Vanguard Study (NICHD, 2013b, p.
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From page 64... ...
This low response rate is problematic in part because if it differs across groups, the resulting estimates based on the NCS Main Study sample may be biased, even with well-designed weighting adjustments. Findings from the initial Vanguard Study recruitment pilot showed variation in cooperation rates at each stage across PSUs and by race and ethnicity, with significantly lower consent for women eligible for recruitment among Asians (Baker et al., 2014)
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From page 65... ...
CONCLUSION 3-6: Assuming that participation in the National Chil dren's Study Main Study follows patterns in the Vanguard Study, the cumu lative response rate to birth for the prenatal stratum would be between 28 and 32 percent, and the rate to age 12 would be 22 to 25 percent -- very small fractions of the eligible sample. The cumulative response rate to age 21 would be 18 to 21 percent.
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From page 66... ...
CONCLUSION 3-8: Enrolling siblings as members of the National Chil dren's Study sample provides many analytic advantages, most prominently the gathering of preconception exposure information for second- and higher-order births. The panel endorses current plans to recruit siblings born after the initially recruited child -- but only within the 4-year recruit ment interval associated with the original primary sampling unit for the target birth -- and to continue to follow these children until age 21.
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From page 67... ...
Design-based estimates of preconception and early pregnancy exposures can be made from the subsequent siblings alone, but as mentioned above, these preconception and early pregnancy findings cannot be generalized to all births, only to second and higher-order births. NEXT STEPS FOR THE SAMPLE DESIGN Overall, the documents given to the panel did not provide sufficient details for an evaluation of whether the proposed sample would meet the minimal standards of a carefully specified, scientifically based sample design required for large national data collections.
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From page 68... ...
This group should be external to the Program Office but would work in collaboration with it on all aspects of the design. The sampling plan needs to include a justification in greater detail for moving away from the geographically based provider-based sampling tested in the Vanguard Study to the currently proposed hospital-based design.
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From page 69... ...
To the extent that some aspects of this design have already been pilot tested, this information should be analyzed to identify gaps that require pilot testing. Results from the Vanguard Study and other pilot testing can be used by the independent survey experts in determining the final study design.
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