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.
Selection diagrams depicting differences between source and target populations. In A, the two populations differ in age (Z) distributions (so S points to Z). In B, the populations differ in how reading skills (Z) depend on age (an unmeasured variable, represented by the open circle) and the age distributions are the same. In C, the populations differ in how Z depends on X. In D, the unmeasured confounder (bidirected arrow) between Z and Y precludes transportability.
