The nation’s jail and prison population decreased in 2015, according to federal data released on Thursday, and the number of adults locked up or on parole or probation fell to a level not seen since 2002 while overall crime continued to drop.

Reasons for the declining incarceration rates include the federal prison system releasing thousands of nonviolent drug offenders in 2015 and states seeking to save money by enacting legislation and policies to reduce prison populations.

In California, for example, Proposition 47 — approved by voters in 2014 — retroactively reduced some drug and property crimes from felonies to misdemeanors. Other states have offered expanded substance abuse treatment programs, established specialty courts and spent more money on re-entry programs aimed at reducing recidivism.

The 2015 data was compiled by the Bureau of Justice Statistics in an annual report that focuses on the nation’s prison and jail populations. Data for 2016 will not be available until next December.