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.
Simulation results for past 30-day prevalences of cigarette use and 95% confidence intervals according to sample size for systematic selection of schools
Note: Prevalence on the y-axis, sample size on the x-axis; * indicates significant difference between selected and excluded samples.
