Chevron USA, Inc. v. Natural Resources Defense Council, Inc., 467 U.S. 837 (1984) (full-text).
The U.S. Supreme Court established a two-prong test to determine the validity of an agency’s construction of a statute. First, if the text and legislative history of the statute demonstrate that Congress has spoken directly on the issue, then that statutory language or history must control. However, under the second prong, if the statute is ambiguous because "Congress has not directly addressed the precise question at issue," the agency's interpretation will stand so long as it is a reasonable one.
Therefore, under Chevron, whether a particular statutory provision is ambiguous or not can change the degree of deference afforded an agency. Where no ambiguity exists, the reviewing court's focus is on the intent of Congress, and it may interpret the law de novo without any deference toward the agency's interpretation. On the other hand, if the statute is ambiguous, either because the language used is susceptible to more than one meaning or because the law contains internal inconsistencies, the reviewing court is not permitted to supplant its own interpretive preferences for that of the agency, unless the agency's interpretation is unreasonable.
Under this deferential standard of review, the discretion available to an agency is inversely proportional to the degree of specificity provided in a particular statute. The less specific a particular bill is regarding an agency's regulatory authority, the more flexibility would be available to her to exercise during implementation.
- ↑ 467 U.S. at 842-45.