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diphtheria. 7
Striking changes in the percentages given above are to be observed in ages 15 and 25,
the proportions noted during the past three years being considerably in excess of the averages
for 1914-18 at those ages. This change is, perhaps, more clearly brought out by the use
of Index Numbers based upon the actual numbers of the cases reported at each age.
{See below).

Having no information as to the numbers living at the various ages, no explanation can be
offered for the changes in incidence indicated by the figures submitted.
Errors of Diagnosis.—Last year the "errors" numbered 55, or 123 per cent, of the cases
notified, showing a great increase above the proportions recorded in 1920 (6'7 per cent.) and
1919 (4*6 per cent.) During the five years 1914-18 the errors averaged 8T per cent.
In addition to the cases marked "errors" there were a fairly large number of cases of
"Bacteriological Diphtheria" of which some information is given in a later section of this
Report.
House, Family Incidence.— During the year 88 cases (including 5 errors) were reported
among the "residents" of institutions within the Borough. The 88 cases were distributed as
set out below :—
Orphanage of Mercy ... ... 10 (all orphans)
Victoria Orphanage ... ... 5 „ „
St. John's Taining School ... ... 8 (girls training)
Paddington Infirmary ... ... 47 (Staff, 3; Workhouse, 2; others,
allocated to home addresses, 40.)
St. Mary's Hospital ... ... 18 (Staff, 9; non-parishioners, 7; others
allocated as before, 2.)
Children's Hospital, Paddington Green ... 5 (all non-parishioners)
The majority of the cases in those institutions were "carrier" cases. After excluding the
54 institutional cases not allocated to home addresses in the Borough and correcting for
"errors," it is found that during the past year there were 357 cases in which the original
diagnosis was not varied which were (apparently) contracted in the homes. The 357 cases
were reported from 301 addresses, 259 of the houses having one case each, 32, 2 cases, 6, 3
cases and 4, 4. The secondary cases, numbering 56, were equal to 15*6 per cent, of reported
cases in which the original diagnosis was not varied. In 1919 the percentage of secondary
cases was 13*6 and in 1920, 13'9. It is, however, thought that little or no importance attaches
to the increase in the proportion of secondary cases observed last year on account of the
numerous instances of "bacteriological diphtheria." The distribution of secondary cases last
year—expressed as percentages of all houses invaded—is compared below with the (average)
distributions for 1914-18 and 1919-20.

Taking the family, and not the house, as the unit, the 357 cases were reported from 306
families, 262 having one case, 39, 2 cases, 3, 3 and 2, 4. The percentage of secondary cases in
families, was 14'8. In 4 instances two cases were reported at the same time.