Talk Left had a link to this lengthy report from the U.S. Sentencing Commission on downward departures from the federal sentencing guidelines.
The report says, among other things:
"Departures play an important role in the federal sentencing guidelines system for several reasons. There may be offense guidelines that do not specify a sentence adjustment for a particular circumstance because either it occurs infrequently in connection with a particular offense, is difficult to quantify, or is truly unique. When such a circumstance does occur, however, it may be important and could be accounted for only by permitting the court to depart from the guidelines.
Departure decisions also provide the Commission with important feedback from courts regarding the operation of the guidelines and improve its ability to make ongoing refinements to the sentencing guidelines. Frequent or increasing use of departures for a particular offense, for example, might indicate that the guideline for that offense does not adequately take into account a particular recurring circumstance."