13 U.S.C. § 181: Population

During the intervals between each census of population required under section 141 of this title , the Secretary, to the extent feasible, shall annually produce and publish for each State, county, and local unit of general purpose government which has a population of fifty thousand or more, current data on total population and population characteristics and, to the extent feasible, shall biennially produce and publish for other local units of general purpose government current data on total population. Such data shall be produced and published for each State, county, and other local unit of general purpose government for which data is compiled in the most recent census of population taken under section 141 of this title . Such data may be produced by means of sampling or other methods, which the Secretary determines will produce current, comprehensive, and reliable data. If the Secretary is unable to produce and publish current data during any fiscal year on total population for any county and local unit of general purpose government as required by this section, a report shall be submitted by the Secretary to the President of the Senate and to the Speaker of the House of Representatives not later than 90 days before the commencement of the following fiscal year, enumerating each government excluded and giving the reasons for such exclusion.