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74
In comparing the instances of recurrence, those which are
separated from their primary cases by the longer intervals can be compared
with cases recurring at shorter intervals only if the respective
primary cases of both classes fall within an identical period.
The recurrent cases which are compared may be widely separated
in time, and indeed where those with intervals of years are compared
with cases separated from their primaries only by months, they
necessarily will fall in different years. In the above table for instance,
the cases recurring after an interval of 4-5 years only, in houses
previously infected are all found in the years 1904-1907.
But the primary cases in the respective houses re-invaded during
these years must have occurred in the years 1900-1903. Each group
of recurring cases therefore which is compared with the group
4-5 years must include those only which are recurrent to primary
cases occurring in the period 1900-1903.

Thus compared it is seen that the cases recurring at intervals
respectively of 1-2, 2-3, 3-4, and 4 to 5 years do not exhibit any
greater liability to be more numerous when separated from their