"Mandatory minimum sentences enacted by Congress played a large part in determining the sentence for these offenders, either outright or through the impact of these statutes on the structure of the guidelines. In fiscal year 2012, crack cocaine offenders and powder cocaine offenders were convicted of an offense that provided for the imposition of a mandatory minimum sentence at rates of 64.0 and 76.4 percent, respectively.6 Powder cocaine offenders obtained relief from a mandatory minimum sentence at a higher rate (32.3%) through the statutory 'safety valve' exception to such sentences, which requires courts to sentence an offender without regard to any otherwise applicable mandatory minimum punishment when certain conditions are met.7 In contrast, 7.6 percent of crack cocaine offenders obtained this relief.
"This difference is largely due to the differing criminal histories of powder cocaine and crack cocaine trafficking offenders. In fiscal year 2012, 58.3 percent of powder cocaine offenders were assigned to Criminal History Category I (offenders with a criminal history score under the sentencing guidelines of zero or one) while only 21.5 percent of crack cocaine offenders were assigned to that category. Only offenders assigned to Criminal History Category I are eligible to receive the benefit of the safety valve.
"Overall, crack cocaine offenders continue to have, on average, a more serious criminal history than any other category of drug offender.8 Crack cocaine offenders were assigned to the most serious criminal history category (CHC VI) in 26.8 percent of all crack cocaine cases. In contrast, powder cocaine offenders were assigned to CHC VI in only 8.8 percent of cases. By comparison, 14.7 percent of heroin offenders were assigned to CHC VI and 10.3 percent of methamphetamine offenders were assigned to CHC VI. Offenders convicted of marijuana offenses generally had the least serious criminal histories, with 66.2 percent assigned to CHC I and only 3.4 percent to CHC VI.9"

Source

US Sentencing Commission, "FY 2012 Overview of Federal Criminal Cases" (Washington, DC: USSC, July 2013), pp. 8-9.
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http://www.ussc.gov/Research_…