Cite as: 516 U. S. 284 (1996)
Opinion of the Court
held in Chapman that § 841(b)(1) requires "the entire mixture or substance . . . to be weighed when calculating the sentence." 500 U. S., at 459. In these circumstances, we need not decide what, if any, deference is owed the Commission in order to reject its alleged contrary interpretation. Once we have determined a statute's meaning, we adhere to our ruling under the doctrine of stare decisis, and we assess an agency's later interpretation of the statute against that settled law. Lechmere, Inc. v. NLRB, 502 U. S. 527, 536-537 (1992); Maislin Industries, U. S., Inc. v. Primary Steel, Inc., 497 U. S. 116, 131 (1990).
Our reluctance to overturn precedents derives in part from institutional concerns about the relationship of the Judiciary to Congress. One reason that we give great weight to stare decisis in the area of statutory construction is that "Congress is free to change this Court's interpretation of its legislation." Illinois Brick Co. v. Illinois, 431 U. S. 720, 736 (1977). We have overruled our precedents when the intervening development of the law has "removed or weakened the conceptual underpinnings from the prior decision, or where the later law has rendered the decision irreconcilable with competing legal doctrines or policies." Patterson v. McLean Credit Union, 491 U. S. 164, 173 (1989) (citations omitted). Absent those changes or compelling evidence bearing on Congress' original intent, NLRB v. Longshoremen, 473 U. S. 61, 84 (1985), our system demands that we adhere to our prior interpretations of statutes. Entrusted within its sphere to make policy judgments, the Commission may abandon its old methods in favor of what it has deemed a more desirable "approach" to calculating LSD quantities, cf. 1995 USSG § 2D1.1, comment., backg'd. We, however, do not have the same latitude to forsake prior interpretations of a statute. True, there may be little in logic to defend the statute's treatment of LSD; it results in significant disparity of punishment meted out to LSD offenders relative to other narcotics traffickers. (Although the number of doses peti-
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