Complex sentence: In recent Censuses, self-identification has been the primary way of identifying race, and presumption of race based on countries or regions given in the Ancestry question is only used when a respondent has answered the Ancestry question but not the Race question.
Split into simpler sentences: In recent Censuses, self-identification has been the primary way to identify race. Presumption of race based on countries or regions given in the Ancestry question is used only when a respondent has answered the Ancestry question but not the Race question.