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.
Propensity score matching for the control group and the case group. A,B The distribution of propensity scores before and after matching between the groups. C,D The distribution of standard differences before and after matching between the groups.
