Another technical problem in comparing statistics on minorities is related to the relatively small number of Asians and Pacific Islanders and American Indians and Alaska Natives. Many socioeconomic and demographic measures are not available for such small population groups. Survey-based measures for these groups often are considered too unreliable to report in statistical publications because the measures are calculated from a small number of respondents. Race and ethnicity are defined by society, not by science. A racial group is often defined according to such physical characteristics as hair type, facial features, and skin color.
Ethnicity usually refers to social and linguistic background and often to physical appearance.
Categories of race and ethnicity vary from one society to another, and they change over time within the same society. The shifting labels and definitions used in the U.S. census reflect the growing diversity of the population and changing political and social climate. Slaves were counted separately. American Indian and Chinese were added as separate categories in 1870. The enthusiasm for scientific methods toward the end of the 19th century helped promote the idea that people could be classified according to distinct biological and physical characteristics, which was reflected in attempts to establish valid mixed-race categories.