The National Children's Study Research Plan A Review (2008) / Chapter Skim
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4 Study Design, Data Collection, and Analysis
4 Study Design, Data Collection, and Analysis
Pages 101-120
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From page 101... ...
A sample cohort of births will be identified from a sample of all noninstitutionalized and cognitively/mentally competent women ages 18-44 who currently live in the national sample of households. The children born to these women during the recruitment period will in turn be followed until they are 21 years old.
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From page 102... ...
to a network of centers tied to multiple study locations and a coordinating center, all of which are separately answerable to NICHD. A contract to serve as NCS coordinating center has already been awarded to Westat, Inc., of Rockville, Maryland, as have contracts to 24 centers, including the seven Vanguard Centers.
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From page 103... ...
Sampling Design NCS sample selection follows well-established principles of population sampling. The most important feature is the strong commitment to random sampling methods in all stages of selection, combined with an effective use of sampling stratification selection with PPS selection in the first sampling stage.
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From page 104... ...
) 1 County/small group of contiguous • 92 roughly equal-sized strata counties • Stratification by county size and Primary sampling unit (PSU)
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From page 105... ...
sites + 98 other "study locations" random selection method -- 18 self-representing PSUs in 13 study -- Measure of size = actual 1999-2002 locations count of the number of births to -- One nonself-representing PSU selected residents in the county(ies) by PPS in each of 92 strata • Rollout in waves: Vanguard first; then 3 random subsets of the remaining study locations • Select one with PPS in each second-stage • One geographic unit per "stratum" stratum -- Measure of size = 1999-2002 estimated number of births • Using a field-verified list of USPS carrier • 10-15 segments selected in most sample addresses, visiting all households, PSUs enrolling all eligible females and resulting births among those who become pregnant during a 4-year sample recruitment and enrollment period (i.e., no subsampling within segments)
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From page 106... ...
A third positive feature is the decision to select the NCS birth cohort sample by screening households for women prior to pregnancy to yield higher sample coverage and avoid the complications of frame multiplicity that result from provider-based approaches (Lesler and Kalsbeek, 1992)
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From page 107... ...
The panel discussed this issue and, recognizing these difficulties, endorses the decision of the NCS staff to omit births to women ages 15-17 from the NCS. Another set of concerns with the NCS sample design is related to sampling within PSUs: The current proposal calls for forming approximately equal-sized segments in each PSU and completely enumerating a random sample of 10-15 of them.
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From page 108... ...
Because of the high cost and less than complete coverage of traditional methods of household listing, the NCS study design calls for an approach that uses vendor-supplied USPS carrier route addresses found in sample segments as a confirmatory source to assist field staff who will be doing within-segment household listing in the traditional way. While this type of adjudicated enumeration of households makes good sense, it should be noted that in our understanding the statistical and practical utility of USPS postal addresses for household listing in area probability samples like the NCS is still in the early stages of development.
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From page 109... ...
While the current design specifications do address which particulate exposures are being considered (NCS Research Plan, Vol.
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From page 110... ...
The coordinating center must provide the centralized release, control, and maintenance of the sample, as well as the uniform training and quality-control activities to ensure a standardized national data collection. The coordinating center must have strong capabilities in locating sample members as well as in data collection itself, in the event that some study centers fail to meet response rate expectations.
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From page 111... ...
The coordinating center must ensure the 39 study centers implement quality assurance procedures, including maximizing response rates, for all aspects of the study. This model requires substantial staff involvement to ensure its success, particularly high-level staff with substantial survey research and data collection skills and experience.
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From page 112... ...
Quality-control procedures, such as random repeat interviews or repeat data collection, while burdensome for study participants, are an important component of quality control to identify poor-quality interview staff and incorrectly coded interviews. Study location production and quality-control results, particularly with regard to response rates, are critical to ensuring a uniform data collection system across the nation.
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From page 113... ...
for the baseline interview, a critical collection for the study, the research plan pays little attention to respondent burden and its impact on the quality of the data. Initial response rates and sample retention are key quality indicators for longitudinal surveys.
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From page 114... ...
While assumptions are based to some extent on the experience of other data collections, incentives to achieve response in other data collections, and the expectation that community involvement will help sample recruitment, the NCS research plan does not explicitly address the best methods and procedures for achieving target baseline response rates. Nor, in any significant way, does it address methods to influence sample units to participate in this long-term study.
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From page 115... ...
Conclusion 4-7: The NCS research plan provides little information concerning best methods for sample recruitment to achieve initial and follow-up target response rates, sample maintenance and sample re tention procedures for implementation at the study sites, community involvement plans consistent with the uniform implementation of data collection procedures, or contingency plans to support study sites that do not achieve target response rates. A successful national data collection program requires the establishment of standards across data collection sites, such as: • A clear set of measurable production and quality goals for each NCS field site.
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From page 116... ...
As part of the plan, the NCS should take advantage of the experience of the Vanguard Centers to evaluate initial enrollment rates, the effectiveness and potential respondent burden of the interview instrument, and the ability of the Vanguard Centers to obtain the required household environmental measures reliably. A program as multifaceted and complicated as the NCS cannot resolve all the methodological issues related to the various types of data collection that will be used over the life of the study.
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From page 117... ...
Conclusion 4-8: The NCS research plan does not address the ongoing methodological needs of the study -- to study data collection procedures and instruments, conduct experiments, and evaluate the quality of the survey operations and the quality of the data -- nor does the plan ad dress the best use of the Vanguard Centers. Recommendation 4-7: To resolve issues that arise during data col lection, the NCS should set aside sufficient resources to maintain an ongoing program of methods research and field experimentation.
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From page 118... ...
• All data sets should include complete documentation on the meth ods used to impute, the assumptions about analyses used when developing the imputation procedures, and the potential strengths and weaknesses of these methods. • To help analysts separate the model-building and model-testing phases of their analyses, it would be helpful if the coordinating center were to use the sample design to develop four independent divisions of the sample and include a variable defining these inde pendent quarter samples in the master NCS data set.
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From page 119... ...
Data from the NCS will provide unprecedented opportunities for the study team and other investigators to learn more about the prevalence of developmental disabilities, the environmental chemicals to which children are exposed, and the relationships between those disabilities and environmental exposures. Past experience with virtually all national data sets is that the research value of the data is maximized when as many skilled analysts as possible are able to access the data for original and replication analyses, and when the peer-review process judges the quality of the analyses performed.
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From page 120... ...
120 THE NATIONAL CHILDREN'S STUDY RESEARCH PLAN • A timeline for making common elements of the data available to the general research community as soon as they have been cleaned and documented. • A plan for analytic support for investigators wishing to use the data, including analysis consultation, documentation of models used for developing unit nonresponse adjustments to the sampling weights and imputation of data elements subject to missingness, and recommended models for adjusting for measurement error or nondetectable levels in environmental variables.
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