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33
increased numbers. This exccss in the next year should appear
in the next succeeding age group, Now when this year of
age reaches the year of maximum incidence of Diphtheria the
combination of excess population for that age group and
relative excess incidcnce of that particular age should be reflected
in a much greater incidence for any particular year.
Considering the year 1927, the years 3 and' 4 are high and
6 also high. Assuming that the age 6 incidence was due
to the disease favouring ago 6 and the increase at 3 and 4 the
increased population at these ages, this would be in accord with
the figures for 1928 which again show a high figure at age 6.
due to age predilection), and' a high figure at 4 and 5 due to
that increased population which in the previous year accounted
for the increase at ages 3 to 4. 1929 would be the year of
conjunction of the exccss population occurring at the age group
most favoured by the disease. This would account for the
particularly high incidence of infection in 1929 of age 6.
1 he figures for Scarlet Fever do not bear this out as, although
for the years 1927 and 1928 it would fit in, 1929 is
aiuomaious
Considering the graphs 2 and' 3 on pages 41 and 43 of the
1928 report, it was found that taking all the years together,
the maximum incidcnce occurrcd during the period 6 to 18
months after the persons became resident on the Estate:
and after that period of time that the incidence fell to below'
normal; further, that the curves were essentially similar for the
two diseases, Scarlet Fever and Diphtheria.
Plotting out the graphs for each year separately for each
disease shows that the type of curve each year differs,, but
that it is essentially similar in each disease. Up to 1927
the figures were low and on charting give an erratic curve.
For 1927 the curves are both flat, with a low maximum
at the period 12 to 18 months, after which it fell gradually.
In 1928 in both diseases the curve shows a peak maximum
after 6 months, falling rapidly after 12 months and
more gradually after 18 months. The 1929 curve is much
flatter. The maximum in Diphtheria is represented by a plateau
for the period of 12 to 18 months. In the Scarlet Fever curve
this is replaced by two maxima at 12 and 24 months, with
a drop at 18 months, Apart from this difference, the curves
resemble each other, there being a rapid fall in the third year,
but after that a gradual decline.