ORDINALSCALE
The ordinal scale arises from the operation of rank-
ordering. Since any 'order-preserving' transformation
will leave the scale form invariant, this scale has the
structure of what may be called the isotonic or order-
preserving group. A classic example of an ordinal
scale is the scale of hardness of minerals. Other in-
stances are found among scales of intelligence, per-
sonality traits, grade or quality of leather, etc.
As a matter of fact, most of the scales used widely
and effectively by psychologists are ordinal scales. In
the strictest propriety the ordinary statistics involving
means and standard deviations ought not to be used
with these scales, for these statistics imply a knowl-
edge of something more than the .relative rank-order
of data. On the other hand, for this 'illegal' statisti-
cizing there can be invoked a kind of pragmatic sanc-
tion: In numerous instances it leads to fruitful results.
While the outlawing of this procedure would probably
serve no good purpose, it is proper to point out that
means and standard deviations computed on an ordinal
scale are in error to the extent that the successive in-
tervals on the scale are unequal in size. When only
the rank-order of data is known, we should proceed
cautiously with our statistics, and especially with the
conclusions we draw from them.
Even in applying those statistics that are normally
appropriate for ordinal scales, we sometimes find
rigor compromised. Thus, although it is indicated in
Table 1 that percentile measures may be applied to
rank-ordered data, it should be pointed out that the
customary procedure of assigning a value to a per-
centile by interpolating linearly within a class interval
is, in all strictness, wholly out of bounds. Likewise,
it is not strictly proper to determine the mid-point of
a class interval by linear interpolation, because the