Jennifer Roche over at the University of Chicago's Becker Friedman Institute:
Studies of incarceration trends and their economic impact are challenging because data collected about prisoners and the flows in and out of the penal system are “very messy,” Neal said. This is because almost 80 percent of all U.S. prisoners are held in state facilities, and the states vary widely in the reliability of the statistics they report.
In a key contribution to the field, Neal and Rick cleaned National Corrections Reporting Program data and ran several consistency checks to identify the states with most complete reporting. From this they focused on seven states with reliable data. They then matched this data with cleaned state-level arrest data to track trends in arrests and prison admissions.
Sound data on arrests, admissions, and releases based on specific crimes allowed them to see changes in prison populations and time served across states and model different outcomes. They held sentencing lengths steady from a baseline of 1985 and asked what the prison population would have been based on just the arrest data. They confirmed that the population would have been much smaller than the current one.
Neal hopes his work will clarify misconceptions about the relative size of the role federal and state policies played in prison population growth. According to Neal and Rick’s estimates, the federal war on drugs—with dramatically harsher penalties for those convicted for possession of crack cocaine, who tended to be black, than for drugs more commonly used by whites—sent approximately 20,000 black men to prison who might not otherwise have gone. But the picture in state prisons is worse.
Their calculations for the seven states in their cleaned data suggest that the black male population of 142,000 is 42.7 percent higher than it would have been under 1985 sentencing policies. Extrapolating that nationwide, their rough calculation adds up to 345,000 “additional” black men held in state prisons under the newer, more punitive sentencing laws.