100
Opinion of the Court
lar factor is within the heartland given all the facts of the case. For example, it does not advance the analysis much to determine that a victim's misconduct might justify a departure in some aggravated assault cases. What the district court must determine is whether the misconduct that occurred in the particular instance suffices to make the case atypical. The answer is apt to vary depending on, for instance, the severity of the misconduct, its timing, and the disruption it causes. These considerations are factual matters.
This does not mean that district courts do not confront questions of law in deciding whether to depart. In the present suit, for example, the Government argues that the District Court relied on factors that may not be considered in any case. The Government is quite correct that whether a factor is a permissible basis for departure under any circumstances is a question of law, and the court of appeals need not defer to the district court's resolution of the point. Little turns, however, on whether we label review of this particular question abuse of discretion or de novo, for an abuse-of-discretion standard does not mean a mistake of law is beyond appellate correction. Cooter & Gell, supra, at 402. A district court by definition abuses its discretion when it makes an error of law. 496 U. S., at 405. That a departure decision, in an occasional case, may call for a legal determination does not mean, as a consequence, that parts of the review must be labeled de novo while other parts are labeled an abuse of discretion. See id., at 403 (court of appeals should "appl[y] a unitary abuse-of-discretion standard"). The abuse-of-discretion standard includes review to determine that the discretion was not guided by erroneous legal conclusions.
IV
The principles we have explained require us to reverse the rulings of the Court of Appeals in significant part.
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