Table 1. Definition of Levels of Evidence.
| Level of Evidence | Definition |
|---|---|
| A | One Level 1 study, two or more Level 2 studies |
| B | One Level 2 study, two or more Level 3 studies |
| C | One Level 3 study, Level 4 or 5 study |
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.