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6
It will be remembered that the Registrar General's estimate
of the population in 1929 was 65,200 and we were asked to use this
figure for 1930. The various rates were calculated upon the basis
of this estimate and it is obvious that the birth-rate and death-rate
given for these years are fallacious. It is important therefore to
revise the population, not only for 1930 but for the other years
between 1921 and 1931. It is difficult though to estimate the population
for the inter-censal years. The usual method is based on the
assumption that the yearly increase has been at the same rate through
out the inter-censal period. The problem is similar to one of compound
interest, the sum of money and the population both increasing
from year to year not merely by the addition of the interest but
of interest on the interest. A series of numbers is said to increase
or decrease in geometrical progression when the successive numbers
differ by a common ratio. It is a useful assumption, that a population
increases in regular geometrical progression, but it is easy
to show how wrong it is in theory, especially in a small district,
where new industries are being established, or where the erection
of houses had proceeded in an irregular manner during the intecensal
period. Although new industries have been established in
this district, and the number of new houses erected was higher
in the second than in the first half of the inter-censal period, the
method mentioned above is as accurate as any other. As probably
it will be the method adopted by the Registrar General it will be
more convenient to adopt it. The population in the ten years
based on this method of calculation would be :—
1921 (Census) 61,299 1926 65,760
1922 62,170 1927 66,700
1923 63,060 1928 67,645
1924 63,945 1929 68,600
1925 64,845 1930 69,565
1931 (Census) 70,560

An examination of these two tables would suggest that the population
in the years 1922— 1925 would be over-estimated, but there
are certain considerations which might modify this view. The
Census of 1921 was not taken until the middle of June and not as