is, on the whole, complete regularity, i. e., heredity is
complete. Neither of these values is ever actually found in
determining coefficients of heredity in the parental relation; these
are usually between 0.3 and 0.5. It should be emphasized again that
this comparison is between whole groups and not between individuals,
and that it fails to allow for the distinction between fluctuations
and true variations. And, further, it should be noted that the
information derived from such a coefficient is defective in that it
takes into account only the relationship between the son and one
parent; the maternal relation is just as important but this has to be
determined separately. There is no satisfactory method of determining
the relation between children and both parents at the same time.

The coefficient of heredity is, therefore, an abstract numerical value
which gives us a fairly precise estimate as to the probable closeness
of the relation between deviations from the group average of any
character in two groups of relatives. The coefficient of _correlation_
is, in general, a measure of the relation between two different
characteristics or conditions in a single group of individuals. The
method of its determination and its limiting values are the same as
for the coefficient of heredity.

By experience the coefficients of heredity and correlation in general
are found to have the following significance:

0.00- no relation.
0.00-0.10--no significant relation.
0.10-0.25--low; relation slight though appreciable.
0.25-0.50--moderate; relation considerable.
0.50-0.75--high; relation marked.
0.75-0.90--very high; relation very marked.
0.90-1.00--nearly complete.
1.00--complete relation.

One further point remains to be considered, which applies not so much
to coefficients of heredity as to coefficients of correlation in
general, i. e., to the relatedness of two different characters or
series of events in a single group of cases or individuals. This is
that coefficients of correlation may be either positive or negative.
That is, the real limits of the value of the coefficient are plus one
and minus one. The example given above of stature of fathers and sons
gives a positive coefficient. Whenever the deviation from the average
of one group is accompanied in the second group by a deviation in the
same direction, the coefficient is positive. A negative correlation
means that deviation from the average i