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.
Solid lines denote actual data. Solid lines without superimposed dashed lines indicate that the best regression line was linear, ie, no change in slope. Dashed lines indicate models that had a significant change in slope at some time point. Year at which the slope changed is shown as a black circle.
