And why we may soon not be able to measure it at all.
Roughly 60 percent of Mexico's federal prison population is serving time for drug crimes.
The power of a questionnaire.
Billions in drug control spending hasn't staunched supply.
The elected leaders of the nation's largest cities are arguing for the right to "set whatever marijuana policies work best" for them.
On Tuesday, voters in those states legalized marijuana for recreational use. But the administration could make implementation very difficult.