The changing attitudes toward marijuana can be difficult to fit into our understanding of crime. At the federal level, the drug is classified as a controlled substance and possession and distribution may carry severe penalties. In most states, possession is a criminal offense with penalties including jail. In a few states, possession is treated as a misdemeanor and in a handful of states, beginning with Colorado and Washington, recreational marijuana possession is now legal. What does this suggest about the difficulties of measuring changes in crime rates over time?