Appendix A
Probability, 1970 Frame
The NORC national probability sample is a stratified, multistage area probability sample of clusters of households in the
continental United States.
6
The selection of geographic areas at successive stages is in accordance with the method of probabilities
proportional to size (p.p.s.). Furthermore, the clusters of households are divided into replicated subsamples in order to facilitate
estimation of the variance of sample estimators of population characteristics.
At the first stage of selection, Standard Metropolitan Statistical Areas (SMSAs) and nonmetropolitan counties covering the
total continental United States were grouped according to size strata within the nine Census regions. All population figures and other
demographic information were obtained from 1970 Census reports. Within each size stratum grouping based upon geographic
location, or racial characteristics (or both), was accomplished before selection. The final frame was further separated into zones or
"paper strata" of equal population size in order to facilitate the selection of replicated subsamples of primary sampling units (PSUs).
7
The selection of PSUs was designed to produce four independent subsamples of equal size. The four subsamples were randomly
combined to form two larger subsamples of 101 PSUs each.
8
The large subsamples are thus internally separable into two replicated
subsamples for variance estimation purposes.
NORC has selected one of the two large subsamples described above to serve as its principal frame of households for the
remainder of the decade. The PSUs fall into 89 distinct SMSAs and nonmetropolitan counties. (New York, a very large SMSA,
represents five PSUs, whereas the smaller counties represent only one PSU.)
The second-stage procedure involved the direct selection of Census block groups or enumeration districts (E.D.s) within
SMSAs or counties, eliminating the traditional intermediate stage of clustering selections within urban places or county division. The
increase in geographic dispersion within the primary areas has a negligible effect on field costs. Before selection, the Census tracts,
minor civil divisions, and Census county divisions containing the block groups and E.D.s were carefully stratified by geographic
location, income, and race, in order to maximize the precision of sample estimation within a PSU. Block groups and E.D.s were then
selected with probabilities proportional to size in numbers sufficient to satisfy survey demands for households expected throughout the
decade. Lists of the separate households contained in the second stage blocks or E.D.s were constructed by field personnel or obtained
from directories. Thus, the principal NORC national probability sample is, in effect, an inventory of identifiable households, each
with a known probability of selection. In a typical sample survey with equal probability of selection for individual households (i.e., a
self-weighting sample), households at which interviews will take place are probabilistically selected from the available lists of
addresses for blocks and E.D.s. The method of probabilities proportional to size results in the assignment of approximately equal
numbers of interviews in each final stage cluster, which in turn leads to increased precision in the estimation of overall population
characteristics.
The NORC national probability frame, with its broad geographic dispersion, its reserves of additional SMSAs and counties,
and its built-in replication, provides sufficient flexibility for application to a wide range of survey tasks. Its design is based on the
consideration of sampling problems that NORC and other organizations have encountered in past surveys, and we believe that it
substantially eliminates many of these difficulties.
Probability, 1980 Frame
1980 National Sampling Frame
The 1980 frame was designed, selected, and listed jointly by NORC and the Survey Research Center. The new frame was
selected, in most cases, in two stages; about one-fifth of the second stage units were subsampled, producing a third stage.
Eighty-four PSUs were selected at the first stage. The PSUs consist of counties, SMSAs, independent cities and, in New
England, parts of counties. Prior to selection, the United States was divided into PSUs; the PSUs were then grouped into 84 strata.
The strata were formed by grouping metropolitan and non-metropolitan PSUs within each of the four Census regions. Within each
region, additional variables were used to define strata. The stratifying variables included within-region geography and size; size was
measured by the 1980 Census count of occupied housing units. One PSU was selected from each stratum using a controlled selection
6
Alaska and Hawaii are not included in this sample.
7
The selection methods used are similar to those described in standard textbooks, e.g., W. E. Deming, Sample Design in Business
Research (New York: Wiley & Sons, 1960), and L. Kish, Survey Sampling
(New York: Wiley & Sons, 1965).
8
In the actual implementation of the selection method, subsamples 1 and 4 resulted in 51 PSUs, whereas subsamples 2 and 3
produced only 50 PSUs. The result was not unexpected and is due to a technical reason, details of which will be provided on request.
The inequality of subsample sizes does not affect the equal probability characteristics of the sample.