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13
The curve of mortality obtained by the method of interpolation adopted in life-table construction
is probably smoother and more regular than the curve which would be obtained were the
actual facts relating to population and deaths at each age known and recorded. It is commonly
believed that certain ages are more critical to life than others, such ages for example as those of puberty
and menopause, but the method of obtaining the life-table curve tends to "smooth out" irregularities
of this nature, if such exist, and the curve obtained can only be regarded as a close approximation to
the true curve of mortality. When the mortality is considered in age groups, however, it is obvious
that the rate of mortality at these age-groups deduced from the life-table figures should closely agree
with the observed rate of mortality, i.e., the rate of mortality calculated directly from the numbers of
the population and deaths upon which the life-table is based. The rate of mortality per unit among
The following table shows the observed rate of mortality at each age-group and the rate of
mortality at the same age-groups calculated from the dx and Px columns of the life-table (males)—
Age-group.
Observed rate of
mortality.
Rate of mortality
from life-table.
Difference
per cent.
0 —
71-97
72-35
+ 0-53
5 —
4-97
4-93
— 0-80
10 —
2-46
2-46
—
15 —
3-47
3-56
+ 2-59
20 —
4-59
4-51
— 1-74
25 —
7-36
7-47
+ 1-49
35 —
14-22
14-30
+ 0-56
45 —
23-14
23-37
+ 0-99
55 —
40-68
4106
+ 0-93
65 —
77-78
78-31
+ 0-68
75 and
upwards
166-45
169-12
+ 1-60
If the rate of mortality in each year of age be calculated from the life table, it will be found
that these rates correspond exactly with the rates shown in the table on pages 9-12; the slight
differences between the observed and the life-table rates shown in the above table are therefore wholly
accounted for by differences in the age constitution of the population comprised in the respective agegroups.
The differences between the age distribution of the life-table population and that of the
actually existing London population will be readily understood when the nature of the two populations
is considered. Thus the Px column of the life table represents a stationary population sustained
by l0 annual births. The age constitution of such a population will necessarily differ from that of an
actually existing population, disturbed by migration and a varying birth-rate, notwithstanding the
fact that the rate of mortality in the two populations is identical. The population and mortality
figures in the preceding tables are graphically shown in diagram I. (see page 1).
Analysis of Tables.
One of the principal objections to the use of death rates for purposes of comparison is the fact
that differences in the age and sex constitution of the population exercise an important influence on
the death rate apart from all questions as to the health conditions affecting the population. In a life
table this source of error is entirely removed, as the mortality per unit at each year of life can be
compared by this means, and the variation in the proportion of persons living at each age in different
populations or in the same population at different times does not affect the accuracy of any comparisons
which may be made. There is, however, another factor which assumes considerable importance when
the mortality in urban communities is made the subject of investigation, and that factor is
"migration." In nearly all large urban communities the age distribution of the population is largely
affected by migration, but the immigration of young adults exercises probably the most important
influence on the death rate. The life table only partially corrects for the factor of "migration." In
so far as migration disturbs the age distribution of the population, its effects are compensated for by
the life-table method of presenting the facts, and no allowance need be made for the effects of migration
in this direction ; but the effects of migration are not limited to a mere disturbance of the age
and sex constitution of the population, inasmuch as the average immigrant, largely recruited from the
rural population, is probably healthier and more robust than the average unit of urban population.
Migration therefore probably alters the character of the population to a considerable extent, and
the method of the life table obviously can make no allowance for the effects of migration in this
direction.
The extent to which the London population is affected by migration may be to some degree
gauged by the figures shown in the following table which are derived from the Census Reports for
1861-1901—