outcome of interest in the same depth. Funding would never be sufficient to do that, nor would the burden on study participants be acceptable. We detail instances in which we think the particular choices made for the NCS merit reconsideration or refinement—indeed, earlier work by NCS working groups and advisers often included options that we think should be on the table but did not appear in the final research plan. Nonetheless, we remain cognizant throughout of the resource limitations for the study, as well as the design constraints imposed by some of the decisions needed to bring the NCS to its current state. At the same time, we note—as does the NCS research plan itself—the opportunity afforded by the study’s length to alter course in one manner or another as appropriate, not only to take account of our comments, but also to take advantage of relevant new measurement techniques and research insights.
The NCS is designed to be a long-running, observational panel study of a nationally representative probability sample of 100,000 births to be followed from before birth to age 21. Data will be collected on multiple exposures and multiple outcomes with repeated measures over time.
The sample design (see NCS Research Plan, Vol. 1, Secs. 6.1-6.4) is multistage and based primarily on households (discussed further in Chapter 4). Omitting some details, the first stage (already completed) was to draw a sample of 110 primary sampling units (PSUs) in 105 locations; the PSUs are counties or groups of counties (or parts of counties in Cook County, Illinois; Harris County, Texas; and Los Angeles County, California), each of which is expected to experience a minimum of 2,000 births during a 4-year enrollment period for the study. The second stage (implemented to date in the PSUs covered by the seven Vanguard Centers) will be to draw a sample of segments in each sampled PSU; the segments are census blocks, groups of blocks or similar subdivisions (some PSUs may be other subunits, such as school catchment areas). A third stage, implemented only in very large segments of sampled PSUs, will be to draw a clustered sample of household addresses. These stages of sampling use stratification on geographic and other variables to ensure representation of population groups (such as minorities) and areas; there is no planned oversampling or undersampling by population groups or areas.
Finally, all of the household addresses in the sampled segments (or clusters) will be visited to identify women who are eligible to participate; such women include those ages 18-44 who are not pregnant as well as all