This paper evaluates the impact of the Federal Sentencing Guidelines on inter-judge sentencing disparity, which is defined as the differences in average nominal prison sentence lengths for comparable caseloads assigned to different judges. This disparity is measured as the dispersion of a random effect in a zero-inflated negative binomial model. The results show that the expected difference between two typical judges in the average sentence length was about 17 percent (or 4.9 months) in 1986-87 prior to the Guidelines, and fell to about 11 percent (or 3.9 months) from 1988-93 during the early years of the Guidelines. We have not sought to measure the effect of parole in the pre-Guidelines period, other sources of disparity such as prosecutorial discretion, or the proportionality of punishment under the Guidelines as compared with the pre-Guidelines era.
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