courts from arrests of adults for drug offenses (also see Figure 11). When combined with these rising adult arrest rates for drug offenses that show no immediate indications of slowing, the recent increases in Q_iS for drug convictions forebode even larger increases expected in the inmate population for drug offenses.

The data from six states illustrate the important role that changing imprisonment policies have played in rising prison populations over the last decade. Increases in the use of imprisonment, usually accompanying explicit policy changes toward harsher penalties, are widespread across states and crime types between 1977 and 1988. Policy changes that involve sentencing guidelines, statutes specifying mandatory minimum prison terms, and restrictions on parole release share a common goal of increasing the certainty and severity of prison terms imposed on offenders. The data suggest that these changes in policy have for the most part resulted in corresponding changes in practice.

Both imprisonment risk following arrest (Q_i) and time served for those committed to prison (S) have increased. Such increases were especially prevalent for rape, where both Q_i and S increased, and for murder and robbery, where time served increased while Q_i remained relatively stable or declined. Increases in the use of imprisonment sanctions also were observed broadly across crime types in California. In Florida, increases in Q_i are more common, whereas increases in S predominate in Michigan, New York, and Pennsylvania. The resulting increases in expected time served per adult arrest, Q_iS, combined with continuing increases in crime rates through the 1980s, contribute to the staggering increases in prison populations of the past decade.

Robbery and drug offenses are distinctive among the crime types examined because they involve opposite changes in Q_i and S. For robbery, increases in time served (S) were dominated by larger declines in imprisonment risk following arrest (Q_i), and robbery was the only crime type among those examined that was characterized by declining or stable levels of expected time served per adult arrest (Q_iS) during the 1980s. In the case of drug offenses, increases in Q_i were often accompanied by declines in S, which moderated somewhat the total increases in Q_iS for this offense. In Florida, the decrease in S results from an administrative program of emergency early release that is implemented as