Basis and modification of definitions

Where appropriate, the definitions in subsection (a) shall be based, with respect to any fiscal year, on the most recent data compiled by the United States Bureau of the Census and the latest published reports of the Office of Management and Budget available ninety days prior to the beginning of such fiscal year. The Secretary may by regulation change or otherwise modify the meaning of the terms defined in subsection (a) in order to reflect any technical change or modification thereof made subsequent to such date by the United States Bureau of the Census or the Office of Management and Budget.