Fig 2. The number of studies undertaken in each country worldwide (grey = 0 studies).
The majority of studies have been undertaken in North America and Europe, followed by South America.
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.