The apportionment of House of Representatives seats is based on the total population, including both citizens and non-citizens, as determined by the decennial census conducted by the United States Census Bureau. The Constitution requires that representatives be apportioned among the states according to their respective numbers, which historically has been interpreted to mean the total population. This includes citizens, non-citizens, and even individuals who are not eligible to vote, such as children and non-citizen residents. (Source: CGPT)