THEORY OF STATISTICS.
than, less than, or equal to the ratio (8, (Anyi Dae). The
whole of the contingency table can be analysed into a series of
elementary groups of four frequencies like the above, each one
overlapping its neighbours so that an rsfold table contains
(r—1) (s—1) such “tetrads,” and the associations in them all can
be very quickly determined by simply tabulating the ratios like
(428,) (Ani 8.) (4,801) (4dnsi Bor), ete., or perhaps better,
the proportions (4.,.8,)/{ (AnB,) + (4118), ete., for every pair
of columns or of rows, as may be most convenient. Taking the
figures of Table II. as an illustration, and working from the
rows, the proportions run as follows :—
For rows 1 and 2. For rows 2 and 3.
1768/2714 0-651 946/1061 0-892
807/2194 0-368 1387/1825 0-760
189/935 0-202 746/1034 0-721
47/100 0-470 53/69 0-768
In both cases the first three ratios form descending series, but
the fourth ratio is greater than the second. The signs of the
associations in the six tetrads are accordingly—
The negative sign in the two tetrads on the right is striking,
the more so as other tables for hair- and eye-colour, arranged in
the same way, exhibit just the same characteristic. But the
peculiarity will be removed at once if the fourth column be placed
immediately after the first : if this be done, 7.e. if “red ” be placed
between “fair” and “brown ” instead of at the end of the colour-
series, the sign of the association in all the elementary tetrads
will be the same. The colours will then run fair, red, brown,
black, and this would seem to be the more natural order, consider-
ing the depth of the pigmentation.
11. A distribution of frequency of such a kind that the
association in every elementary tetrad is of the same sign
possesses several useful and interesting properties, as shown in
the following theorems. It will be termed an isotropic dis-
tribution.
(1) In an isotropic distribution the sign of the association is
the same not only for every elementary tetrad of adjacent Jrequen-
cies, but for every set of four frequencies in the compartments
common to two rows and two columns, e.g. (4,B,), (dn.,B,)
(4dnBrio) (drei pBrig):
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