FIGURE 1.
An illustration of a natural sampling framework: the total population (100) is categorized into groups (5/95) and those groups are categorized into parallel sub-groups below that.
Official websites use .gov
A
.gov website belongs to an official
government organization in the United States.
Secure .gov websites use HTTPS
A lock (
) or https:// means you've safely
connected to the .gov website. Share sensitive
information only on official, secure websites.