pattern of housing in the particular parts of the country that are selected.1 Some key features of the NCS sample design are presented in Table 4-1. The use of established random selection methods in each sampling stage will ensure that the NCS samples of households, eligible women of childbearing age, and births are national probability samples.
In the first stage, 110 individual counties or small groups of contiguous counties were randomly chosen (by staff at a federal agency partner, Centers for Disease Control and Prevention/National Center for Health Statistics) to serve as the set of primary sampling units (PSUs). Each of these PSUs was selected with probability proportional to size (PPS), with the actual 1999-2002 count of live births serving as the measure of size for each PSU. Since multiple PSUs were chosen in some of the largest metropolitan areas, the 110 PSUs are found in 105 different “study locations or sites.” Fieldwork will begin in two waves (in 2009 and 2011) on random subsets of the remaining 98 study locations (the Vanguard Centers will undertake sampling, participant enrollment, and data collection in selected PSUs beginning in mid-2008). Within each sample PSU, 10-15 small groups of neighboring census blocks or block groups, called “segments,” are to be randomly chosen. All children born to all eligible female residents of all households located in the 1,100-1,650 sample segments will be included in the study, implying that no subsampling is to be done within sample segments. To improve the efficiency of screening for pregnancies among the women ages 18-44 who are identified in selected households, women who are more likely to become pregnant will be monitored more intensively than the other women. These women may also be recruited through the health care providers they visit during pregnancy and delivery.
Data collection and other survey process activities are to be completed using a common National Institutes of Health organizational structure, in which direct governance is provided by program staff of the National Institute of Child Health and Human Development (NICHD) 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. NICHD anticipates the need for about 13 to 15 additional centers.
Developed within this organizational framework, the field operations plan prepared by Westat includes some elements of standardization and
While only two stages of selection are described here, additional stages may be added as needed to accommodate the largest primary sampling units (consisting of more than 500 segments) and high-density urban segments with high-rise residential buildings and other features that make it difficult to achieve targeted household sizes for that stage of selection.