:It's an interesting concept, definitely. I've never filled out a census before, but I wonder how the question is worded when asking about ancestry. To me, American ancestry means Native American, or relation to the peoples of America prior to European discovery. Are people making mistakes in filling it out (or answering it in a way not intended by the Census Bureau) or is this a legitimate option for answering this question?  
 :In reading the lead to the culture section, it does seem that race is a better word to use, though I don't say that because American ethnicity exists; I say that because America as a nation has no one race, and there is no "American race". North America may have (or better phrased, had) races, but once you put political boundaries on the map and consider only the United States, then no, we don't have a race. We do have at least some sort of ethnicity, though I'm not sure it's worth rewording the section just to include a link to that article. ''''''