Complex sentence: The "residual method" is widely used to estimate the illegal immigrant population, where the reported census number of self proclaimed foreign born people in the U.S. census is subtracted from the known number of legal immigrants to obtain the unauthorized immigrant (residual) population.
Split into simpler sentences: The "residual method" is widely used to estimate the illegal immigrant population. With this method, the reported census number of self-proclaimed foreign born people in the U.S. census is subtracted from the known number of legal immigrants to obtain the unauthorized immigrant (residual) population.