efforts to track mixing between groups led to an earlier proliferation of historical categories (such as "mulatto" and "octaroon" among persons with partial african descent) and "blood quantum" distinctions, which became increasingly untethered from self-reported ancestry. in the 20th century, efforts to classify the increasingly mixed population of the united states into discrete categories generated many difficulties (spickard 1992). by the standards used in past censuses, many mixed-race children born in the united states were classified as of a different race than one of their biological parents. in addition, a person may change personal racial identification over time because of cultural aspects, and self-ascribed race can differ from assigned race (kressin "et al." 2003).