Most numerical data on ethnicity and "race" in the U.S. derive from national census data gathered every 10 years. Unfortunately, even the latest census is flawed due to inconsistent and incomplete data collection. It is likely that some urban minorities and migrant farm workers are undercounted. In addition, people have been counted in terms of ethnicity and "race" mostly as a result of their own self-disclosure as to which categories they fit, and they had to choose from the limited list specified by the national government. The failure to allow people to identify with categories that they themselves subjectively volunteer makes the data less reliable.
The U.S. Census Bureau considers some group differences to be racial and others to be purely ethnic. Specifically, they make an ethnic distinction between Hispanic and non-Hispanic. In contrast, categories such as Chinese and Vietnamese are considered racial.