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.
Vertical lines represent the standard error (n = 30), and values for the same date with the same letter are not significantly different at a 5% significance level.
