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31
straight line as did the Scarlet Fever curve, bends over parallel
to the population curve This, of course, may be just
chance. As already indicated, the incidence rate which is low
would have been normal had some of the surplus cases of early
1930 occurred in late 1929. Again, too, accounting for the
different lines of the expected curves, whereas for the country
the rate of Diphtheria per 1,000 population in 1929 showed
an increase of only 2 per cent. over that of 1928, the Scarlet
Fever rate increased by 18 per cent.
A study of the graphs as they stand, however, shows that
up to this year they are essentially similar for live two diseases,
though with a longer period of lag in Diphtheria before the
excess incidence appeared; but that when it did appear, the
surplus was relatively greater than for Scarlet Fever; suggesting
that the same factors arc in operation in both rases. If 'this
is so, then presumably the alteration in the present year is
due to the change occurring in the case of one before the.
other, just as the increase occurrcd in Scarlet Fever earlier than
Diphtheria. If this be the trend, then shortly, the incidence
curve should cross the exported curve again to come to lie
below it—that is, the incidence of Diphtheria will be relatively
lower than that obtaining in the country as a wholes
Similar figures held as shown previously with regard to
the time persons had been resident in the area before suffenng
from infection. In the case of Diphtheria, out of 187 primary
cases 7 occurrcd within the first five months, whereas 77 look
place between this period and 18 months, leaving only 103
occurring in persons who had been here over 18 months. Similarly
with regard to Scarlet Fever, 113 cases occurred between live
and 18 'months of the patient's coming to reside in the area
17 occurred within the five months and 192 over the 18 months.
This disproportionate distribution might conceivably be
due to two factors.
1, The development of the. district.
Only a 'few houses have been up seven years, whereas most,
at the time of calculating, would have been up six months.
Naturally, therefore, a greater numerical incidence occurs
amongst those houses which have been up the shorter time
owing to their greater number.