Figure 1.
Preferred reporting items for systematic reviews and meta-analyses flow diagram. CPT = cigarette purchase task. See the online article for the color version of this figure.
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.