"Interviews with prosecutors and defense attorneys in thirteen districts across the country revealed widely divergent practices with respect to charging certain offenses that triggered significant mandatory minimum penalties. These differences were particularly acute with respect to practices regarding filing notice under section 851 of title 21 of the United States Code for drug offenders with prior felony drug convictions, which generally doubles the applicable mandatory minimum sentence. In some districts, the filing was routine. In others, it was more selectively filed, and in one district, it was almost never filed at all.12 Our analysis of the data bore out these differences. For example, in six districts, more than 75 percent of eligible defendants received the increased mandatory minimum penalty for a prior conviction, while in eight other districts, none of the eligible drug offenders received the enhanced penalty.13
"Similarly, the Commission’s interviews revealed vastly different policies in different districts in the charging of cases under section 924(c) of title 18 of the United States Code for the use or possession of a firearm during a crime of violence or drug trafficking felony. In that statute, different factors trigger successively larger mandatory minimum sentences ranging from five years to life, including successive 25-year sentences for second or subsequent convictions. The Commission found that districts had different policies as to whether and when they would bring charges under this provision and whether and when they would bring multiple charges under the section, which would trigger far steeper mandatory minimum penalties.14 The data bears out these geographic variations in how these mandatory minimum penalties are applied. In fiscal year 2012, just 13 districts accounted for 45.8 percent of all cases involving a conviction under section 924(c) even though those districts reported only 27.5 percent of all federal criminal cases that year. In contrast, 35 districts reported 10 or fewer cases with a conviction under that statute.
"When similarly situated offenders receive sentences that differ by years or decades, the criminal justice system is not achieving the principles of fairness and parity that underlie the SRA [Sentencing Reform Act]."

Source

Statement of Judge Patti B. Saris, Chair, United States Sentencing Commission, For the Hearing on “Reevaluating the Effectiveness of Federal Mandatory Minimum Sentences” Before the Committee on the Judiciary, United States Senate, September 18, 2013, pp. 3-4.
http://www.ussc.gov/Legislati…