Census net migration data and estimates of net migration obtained by the residual method, representing the difference between total population change and natural change during a period, are compared, and some general problems in the use of the residual method are discussed. Several residual methods—the vital statistics method and the forward, reverse, and average survival rate methods—are described, compared, and evaluated. On the basis of a symbolic model representing the population in an age group in terms of migration cohorts, it is shown how the various survival rate formulas, unlike the vital statistics method, fail to make an accurate allowance for the net migration of persons who die during the migration period, except under very restricted conditions of migration. The maximum theoretical errors in the use of the various survival rate formulas, resulting from the inability of survival rates to measure deaths occurring in an area exactly, and the theoretical errors under different conditions of timing of migration, are developed, and suggestions are given as to how the problem may be handled.