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The value of this graph representing, for each year, the local
incidence compared with that obtaining in the country as a whole,
depends on the accuracy of the previous calculations; namely,
those in which one arrives at that unknown quantity, the number
of children under 15. The type of graph, however, is of much more
importance than its actual level and the type will not be invalidated
by any inaccuracies which arc the same in each succcssive year.
For instance, should the population per house be 5.0 instead of 4.9;
this would increase the total susceptible population by 5% and
so would lower the incidence per unit of susceptible population by
the same amount, and cause the graph of comparison of local to
general incidence to lie nearer the base line.
There is, of course, the possibility, and indeed likelihood, of
two changes having occurred which would affect the figures
recorded. The first is that the average per house in the earlier
days was slightly less than at the present time, the existing figure
being an average of houses of which some have only recently been
erected, others built and inhabited a number of years. These older
houses will, on an average, possess a larger population, owing to the
families being increased by births, but not correspondingly
diminished by deaths. Moreover, the tendency seems to be
increasing of bringing down the relatives of one of the parents of
the family. This effect, in general, would be slight, and, at most,
would result in a slight elevation of the curve for the first few
years. The other factor is that concerning the age distribution
of the population. It has been assumed that, for each of the years,
the percentage population under 15 is 45 of the total. Ordinarily,
this figure would, with the passing of years, tend to diminish, as
each year would see a number of children passing from the under
15 to the over 15 group. Moreover, the other factor previously
Mentioned, viz., the importation of elderly adults, would act in the
same direction. These influences would, however, be balanced to
a great extent by the new births in the area; a number greater
than the number of children passing from 14 to 15. The balance
these effects probably results in that assumption being tolerably
accurate for each year.