19 CFR 151.70: Method of sampling by Customs.

A general sample shall be taken from each sampling unit, unless it is not feasible to obtain a representative general sample of the wool or hair in a sampling unit or to test such a sample in accordance with the provisions of § 151.71. At the request of the importer, two general samples may be taken from a sampling unit if the taking and testing of a second general sample is feasible. If two general samples are taken, one general sample shall be held for use in making a second test for clean yield if such a test is requested in accordance with the provisions of § 151.71(c), or if a second test is found desirable by the Center director or the chief chemist.