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Population.
It is impossible to state exactly the number of inhabitants in any given
district at any given time without taking a census. In this country a census
is only taken once in every ten years, and therefore the population of any place
in any one of the other nine years is, of necessity, a matter of more less
accurate guess-work. It is difficult to make a correct estimate of the numbers
inhabiting districts, and this difficulty increases, and the margin of error
becomes greater, the longer the period that has elapsed since the last census.
In small towns and localities that are practically at a standstill, neither
developing nor decaying, any increase of population is due to the normal excess
of births over daaths, and a fairly accurate estimate can ba made. But in
districts like this where houses are being built, new roads opened out and estates
developed, there is an artificial as well as a natural increase which has a very
material effect on the accuracy of any estimate. There are several methods
used for estimating populations in periods between censuses, but there is no
fixed rule as to which should be employed, and this fact should be taken into
account in comparing the vital statistics of one district with another on the
figures given by Medical Officers of Health in their annual reports. Actual
numbers of births, deaths, and cases of infectious disease, etc., are correct, or
should be, because they are mere records of facts; but all rates, such as those
of births and deaths (with the exception of the infantile death-rate) are calculated
on the number of inhabitants and will vary with the estimate of
population.
Of the various ways of estimating, two are most frequently used. The first
is the method used by the Registrar-General, and may be called the official one.
It is made by a mathematical calculation based on the assumption that the
same rate of increase that took place between one census and the next will be
maintained in the following years. It calculates the increase of population at
compound interest, but makes no allowance for fresh capital brought in.
The second method is simpler in theory though more difficult in practice,
but the result is probably more accurate. It is based on the number of occupied
houses at the middle of the year, and the average number of people living in
each house.