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.
One-headed arrows indicate the direction of hypothetical regression. Two-headed arrows indicate hypothetical correlation, which are actually among error components of corresponding observed variables, but the error components were omitted for the sake of simplicity. Letters (a to n) indicate hypothetical paths as described in the main text.
