7 years. The child
mortality (first 4 years of life) was 7.5% of the total mortality, 69
families showing no deaths of that kind. The group is as a whole,
therefore, long-lived.
The problem was to measure the resemblance between brothers and sisters
in respect of longevity,--to find whether knowledge of the age at which
one died would justify a prediction as to the age at death of the
others,--or technically, it was to measure the fraternal correlation of
longevity. A zero coefficient here would show that there is no
association; that from the age at which one dies, nothing whatever can
be predicted as to the age at which the others will die. Since it is
known that heredity is a large factor in longevity, such a finding would
mean that all deaths were due to some accident which made the
inheritance of no account.
In an ordinary population it has been found that the age at death of
brothers and sisters furnishes a coefficient of correlation of the order
of .3, which shows that heredity does determine the age at which one
shall die to considerable extent, but not absolutely.[190]
The index of correlation[191] between the lengths of life within the
fraternity in these 100 selected families, furnished a coefficient
of-.
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