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.
Proportion of participants who achieved at least a given number of consecutive breath CO samples <3 ppm across each group. Open triangles, filled circles, and gray triangles represent escalating reinforcement, fixed reinforcement, and control participants, respectively.
