ling
with the statistics of disease has taken at least the first step towards
sanity by grasping the fact that as an attack of even the commonest
disease is an exceptional event, apparently over-whelming statistical
evidence in favor of any prophylactic can be produced by persuading the
public that everybody caught the disease formerly. Thus if a disease is
one which normally attacks fifteen per cent of the population, and if
the effect of a prophylactic is actually to increase the proportion to
twenty per cent, the publication of this figure of twenty per cent will
convince the public that the prophylactic has reduced the percentage by
eighty per cent instead of increasing it by five, because the public,
left to itself and to the old gentlemen who are always ready to
remember, on every possible subject, that things used to be much worse
than they are now (such old gentlemen greatly outnumber the laudatores
tempori acti), will assume that the former percentage was about 100. The
vogue of the Pasteur treatment of hydrophobia, for instance, was due
to the assumption by the public that every person bitten by a rabid dog
necessarily got hydrophobia. I myself heard hydrophobia discussed in
my youth by doctors in Dublin before a Pasteur Institute existed,
the subject having been brought forward there by the scepticism of an
eminent surgeon as to whether hydrophobia is really a specific disease
or only ordinary tetanus induced (as tetanus was then supposed to be
induced) by a lacerated wound. There were no statistics available as to
the proportion of dog bites that ended in hydrophobia; but nobody ever
guessed that the cases could be more than two or three per cent of the
bites. On me, therefore, the results published by the Pasteur Institute
produced no such effect as they did on the ordinary man who thinks that
the bite of a mad dog means certain hydrophobia. It seemed to me that
the proportion of deaths among the cases treated at the Institute was
rather higher, if anything, than might have been expected had there been
no Institute in existence. But to the public every Pasteur patient
who did not die was miraculously saved from an agonizing death by the
beneficent white magic of that most trusty of all wizards, the man of
science.

Even trained statisticians often fail to appreciate the extent to
which statistics are vitiated by the unrecorded assumptions of their
interpreters. Their attention is too much occupied with the