Most persons who have dealt with mortality experiences have, no doubt, observed the variation in the average rates deduced according to the grouping of the exposures, but a few observations thereon, for the benefit of students of life contingencies, may, perhaps, be acceptable.

The first process in the investigation of the rate of mortality among lives assured in an office, or a special class, as annuitants, is to obtain the number exposed to risk of mortality, and the deaths which have actually occurred, at each age, during a certain period of time, as, say, the past five years, or since the company was established. The method to be used for deducing the numbers exposed to risk will depend on the special circumstances of the experience under observation, and need not here be discussed; but, assuming the exposures and deaths to have been ascertained, the mortality experience of the company is deduced by dividing the number of deaths at each age by the corresponding exposures.