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.
We consider two categories: Clusters P1, P2, and P3 as one, and Clusters P4, P5, and P6 as the other one. For the first category, there is only one huge spike during popularity evolution. For the second category, there are multiple spikes.
