It follows from this hypothesis that if the smoking
mothers in the study had not acquired the smoking habit,
their infants would have been distributed by weight and
survival as were the infants of the nonsmoking mothers in
this study. It is therefore possible to determine for each
birth weight group the number of births and deaths which
would have occurred to the smoking mothers if they did
not smoke. We refer to these as the “expected” as distinguished
from the “displaced,” births. The excess of the
observed numbers of births and deaths among smokers
over these expected numbers provides an estimate of the
magnitude and early mortality of the births which were
presumably “displaced” because of mother’s smoking.