The effect of relaxing the conditions regarding the equality of the mortality rates and of the errors of enumeration between the states and the nation, on the error in the estimation of the net migration rate by the census survival ratio method, is examined and it is shown that, when the national population is closed, the necessary and sufficient condition for this error to become zero is that the relative degree of enumeration for any agexof a given sex in theith state as compared with the degree of enumeration in the nation at the first census bears the same ratio to the relative degree of enumeration for the same cohort at age (x+n) at the second census, as the survival ratio of that cohort from agexto (x+n) in theith state bears to the corresponding survival ratio in the nation as a whole.