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In the last series it is assumed that the death-rate is
4 per cent. Here with a birth-rate of 104, and an increaserate
of 100, the mean duration of life is 4.70 years; and with
a birth-rate of 4 per cent. and a stationary population 25
years. With a birth-rate of .5 per cent, and an annual loss
of 3.5 per cent, of the population the duration of life mounts
up to 58.37 years; while with a birth-rate of .05 and an
annual loss of 3.95 it rises to 108.73.
It may be added that in all cases, as well as in the
first, the increase of the birth-rate to infinity reduces the
mean duration of life to zero, and the reduction of the birthrate
to zero raises the mean duration of life to infinity.
The table shows further the degree in which, under
the same rates of increase, the mean duration of life varies as
the death-rate varies. Thus, with an increase of 100 per
cent. the mean duration of life, which is 7.6 when the deathrate
is .5 per cent., becomes successively 6.66, 6.08, 5.67,
5.35, 5.10, 4.89, and 4.7, as the death-rate increases by
increments of .5 up to 4 per cent.; and with a stationary
population, under similar conditions of the death-rate, the
duration of life is 200, 100, 66 2/3, 50, 40, 33 1/3, 28.57, and 25.
The table shows also the degree in which, under the
same birth-rates, the mean duration of life becomes shorter
as the death-rate increases. Thus, assuming the birth-rate to
be 4 per cent., the average duration of life becomes, with a
death-rate of .5 per cent. 60.4 years; with a death-rate of 1
per cent. 46.9; with a death-rate of 1.5, 39.74; with a
death-rate of 2, 35 ; with a death-rate of 2.5, 31.56; with a
death-rate of 3, 28.91; with a death-rate of 3.5, 26.77; and
with a death-rate of 4, 25, and assuming a birth rate of .05