2. The rate of mortality, The mortality, or rate of mortality, is the number of deaths which takes place in a given population, in a given space of time. The calculation is gene rally made for a year, so that the ratio conies to express the number of the living out of which one will die annually. The ratio is sometimes stated as a fraction, and sometimes as a percentage proportion. If, for in stance, out of a living population of 100,000, two thousand deaths take place every year, or -a, or 2 per cent. is the mortality, or rate of mortality, to which that population is exposed.
In estimating the value of this test, it is necessary to bear in mind that the rate of mortality varies fir every year of life. It follows, therefore, as a natural consequence, that the rate of mortality, like the mean age at death, must be materially influenced by the ages of the living population. In a po pulation containing a large proportion of young children, subject to a very high mor tality, the aggregate rate of mortality for the entire population will be necessarily higher than in a population abounding in older per sons and having a comparatively small number of children. But a very cursory examination of tables of mortality will convince us that the error attaching to the rate of mortality as a test or measure of the sanitary condition of a population is much less than that which is inherent in the mean age at death ; for not only do the extremes of life approximate much more closely to each other in their respective rates of mortality than in the mean ages at death, but ages which, though less widely separated, are far enough apart to affect the mean age at death, are exhibited as subject to a mortality very nearly identical. From 5 to 10 years of age, for example, the rate of mortality, in the male population of England, is 070 per cent., and from 20 to 30 years of age 974 per cent.; so that, while out of 100,000 persons dying at the respective ages of 5-10 and 20-30, 970 and 974 persons would die in the year, their age at death would count in the one case as something between 5 and 10, and in the other at some age between 25 and 30. The difference in the rate of mor talky would be almost nothing, while the dif ference in the age would amount to about 20 years. So also if we compare individual years instead of terms of years. The mor tality at 10 years is 91 per cent., at 20 years 784 per cent. ; at 11 years it is .702 per cent., at 18 years .709 per cent. The rate of mor tality differs very slightly in the ages brought under comparison, while the mean age at death differs by 10 and 7 years respectively.
These d priori reasonings are fully borne out by the results of actual comparison. In one of the reports of the Registrar General* the expectation of life (which will be presently shown to be the true test or measure of the sanitary state of a population), the mortality, and the mean age at death, for six different populations, are compared with each other, with what result the following table will show.
Expectation of life, in years : Surrey, 45 ; England, 41; France, 40; Sweden, 39; Metro polis, 37 ; Liverpool, 26.
Rate of mortality, or one death in Surrey, 52 ; England, 41; France, 42; Sweden, 41; Metropolis, 39 ; Liverpool, 30.
Mean age at death, in years: Surrey, 34; France, 34; Sweden, 31 ; England, 29; Me tropolis, 29; Liverpool, 21.
The rate of mortality, then, keeps pace with the expectation of life, to such an extent, at least, as to place the six communities in the same relative position ; while, according to the mean age at death, the nations which stand third, fourth, and fifth on the list suffer transposition, Frarice and Sweden taking rank before England in the scale of salubrity, and England, which holds, of right, the second place immediately after her own county of Surrey, is made to descend to the fourth rank. When compared, therefore, with an accurate measure of the duration of human life, the rate of mortality shows itself more worthy of confidence than the mean age at death. Nevertheless, for reasons already assigned, it must not be looked upon as altogether free from objection, and must, in every case, be regarded as of inferior value to the test next to be considered, namely, 3. The expectation of 11e. The expectation of life, or the mean future life-time, is the mean number of years which, at any given age, the members of a community, taken one with another, may expect to live. This ex pectation is embodied in those series of cal culations which are technically known as life tables, and which are so largely in use in the important operations of life-assurance. When correctly calculated, they are based on the two concurrent series of facts, the numbers and ages of the living and the numbers and ages of the dying, and they therefore comprise the two elements necessary to perfect ac curacy. When based upon the ages of the dying alone, they are open to nearly the same objection as those which apply to the mean age at death.