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.
In (a) locations sampled, triangles indicate sites where fields were sampled, and the sampling scheme (b) illustrates how a field and accompanying margin were sampled.
