times been called intensive, and both interval and
ratio scales have sometimes been labeled extensive.
It will be noted that the column listing the basic
operations needed to create each type of scale is cumulative:
to an operation listed opposite a particular scale
must be added all those operations preceding it. Thus,
an interval scale can be erected only provided we have
an operation for determining equality of intervals, for
determining greater or less, and for determining equality
(not greater and not less). To these operations
must be added a method for ascertaining equality of
ratios if a ratio scale is to be achieved.
In the column which records the group structure of
each scale are listed the mathematical transformations
which leave the scale-form invariant. Thus, any numeral,
x, on a scale can be replaced by another numeral,
x', where x' is the fanction of x listed in this column.
Each mathematical group in the column is contained
in the group immediately above it.
The last column presents examples of the type of
statistical operations appropriate to each scale. This
column is cumulative in that all statistics listed are
admissible for data scaled against a ratio scale. The
criterion for the appropriateness of a statistic is invariance
under the transformations in Column 3.
1 A classification essentially equivalent to that contained
in this table was 1rr.-. ut4.*d bl4efor0e (te International Congress
for the Unity of Scitlncet .Sc',teiul.r l141. The writer is
indebted to the la tr Pror'. (.;. D. Birkl;,iffl for a stimulating
discussion which led to the completion of the table in essentially
its present form.
of variation remains invariant only under the similarity
transformation (multiplication by a constant).
(The rank-order correlation coefficient is usually
deemed appropriate to an ordinal scale, but actually
this statistic assumes equal intervals between successive
ranks and therefore calls for an interval scale.)
Let us now consider each scale in turn