Figure 1.
Source counts obtained after each of the three major stages of literature review (circular arrow represents addition of new full text-reviewed sources from reference lists of prior full text-reviewed sources).
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.