distributions by geographic division, household income, Hispanic origin, and racial identification. By
design, this sample of 1799 responding RDD households represents all 110 million households in the
U.S., except for about 30,000 that fall into the second stratum. The second stratum was randomly
sampled from a population list of telephone numbers for households known to the National Parks
Foundation (NPF) to have purchased a National Parks Pass (NPP) between April 2004 and March 2005
(i.e., from one to two years prior to the survey). This sample was also pre-stratified by geographic
division, and the 1974 responding NPF households were post-stratified to reflect the geographic
distribution of the NPF population list.
The total sample size across both strata was 3773 households. Within-household sampling was not
undertaken, because the relevant unit of analysis is the household. The NRP (like the GEP and NPP) is
not an individual pass; rather, it entitles the members of a household to access federal lands for recreation.
Therefore, an adult respondent in each household included in the survey was asked to provide information
on behalf of the household.
With total NPP sales of approximately half a million passes sold per year, the NPF population list of
only 30,629 households covers only a small subset of NPP purchasers; obviously, it covers an even
smaller fraction of all U.S. households. The NPF population, as well as the NPF sample drawn from that
population, mainly represents households that purchased the pass on-line. The NPF sampling frame
omits the much larger number of households that purchased the NPP in-person at a park, except for those
that chose to provide their phone numbers by mailing a reply card to NPF, requesting a park “owner’s
manual,” or sending the NPF a donation. It also omits the roughly 50,000 annual purchasers of the GEP.
Results from the NPF sample are of interest because they reflect a small but noteworthy group of
supporters of the national parks, while the RDD sample is designed to be representative of the full
spectrum of the population of U.S. households.
Both samples were screened, through questions early in the survey, to eliminate from our analysis
those households in which anyone qualified for a Golden Age or Golden Access Passport. These lifetime
passes, available only to senior citizens and the disabled, effectively remove a household from the market