Figure 2.
Assessment of risk of bias in the individual studies. Green (+) symbol = low risk of bias; yellow (?) symbol = unknown; red (–) symbol = high risk of bias.
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.