Sampling design overview
Stratified random sampling
1. Stratified random samples according to sex, race/ethnicity, and
socioeconomic status (measured by regionally specific, US Housing and Urban Development HUD adjusted
income of the parents as a surrogate variable) were obtained at each
PSC in order to match total sample proportions of these factors, marginally
across all PSCs, to those of the entire U.S. population as determined by cross-tabulated US Census 2000 family income by race/ethnicity proportions.
2. In order to address concerns regarding a “super-normal”
sample population, we analyzed groups of data stratified according to
the full-scale WASI IQ of the subject are available to also match nationwide IQ distributions.
Age targets, limits, and groups
Target ages were staggered at entry with 20% compounded attrition for
both Objectives; please see the Sampling Strategies section in the NIHPD
Protocol for more detail.
Age at study entry
Objective 1: 4 years 6 months (Minimum) - 18 years
3 months (Maximum)
Objective 2: 10 days (Minimum) - 4 years 5 months
(Maximum)