can be statistically palmed off as a magic-spell conferring all
sorts of privileges.

In the case of a prophylactic enforced by law, this illusion is
intensified grotesquely, because only vagrants can evade it. Now
vagrants have little power of resisting any disease: their death
rate and their case-mortality rate is always high relatively to
that of respectable folk. Nothing is easier, therefore, than to
prove that compliance with any public regulation produces the
most gratifying results. It would be equally easy even if the
regulation actually raised the death-rate, provided it did not
raise it sufficiently to make the average householder, who cannot
evade regulations, die as early as the average vagrant who can.


THE SURPRISES OF ATTENTION AND NEGLECT

There is another statistical illusion which is independent of
class differences. A common complaint of houseowners is that the
Public Health Authorities frequently compel them to instal costly
sanitary appliances which are condemned a few years later as
dangerous to health, and forbidden under penalties. Yet these
discarded mistakes are always made in the first instance on the
strength of a demonstration that their introduction has reduced
the death-rate. The explanation is simple. Suppose a law were
made that every child in the nation should be compelled to drink
a pint of brandy per month, but that the brandy must be
administered only when the child was in good health, with its
digestion and so forth working normally, and its teeth either
naturally or artificially sound. Probably the result would be an