The mortality between adjacent age classes can be estimated from the survivorship schedules derived in Chapter 3. The data, however, differ in each case in the number, size, and age limits of the age classes; therefore, it is not possible to derive strictly comparable information from the data sets. Further, it is not yet possible to define the internal mortality structure of each age class which must be known for the computation of many elements of the life table.

If we assume, as is commonly done in demography, that human mortality patterns follow some function of age, then we can fit our 50 source populations to that function and assume that deviations from that fit are merely stochastic. This process of curve fitting is called graduation, and it is a powerful and useful smoothing operation for mortality or survivorship data.