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.
Pie chart sizes of 2013 and 2014 are in proportion according to the respective numbers of citations. Searches were made in September 2015 (‘Update September 2015’ sheet).
