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.
(a) Common keywords cluster analysis (various colors show various types, the circle's size manifests the keywords frequency, and line's width manifests the link intensity). (b) Common keyword evolution trend over time (the blue and yellow boxes show the earlier and latest keywords, respectively).
