THE MISMEASURE OF MANcalled oblique simple structures, or systems of correlated axes.
(Uncorrelated axes are called "orthogonal" or mutually perpendic-
ular; correlated axes are "oblique" because the angle between them
is less than 90°.) Just as several methods may be used for determin-
ing orthogonal simple structure, oblique axes can be calculated in
a variety of ways, though the object is always to place axes within
clusters of vectors. In one relatively simple method, shown in Fig.
6.11, actual vectors occupying extreme positions within the total set
are used as factor axes. Note, in contrasting Figs. 6.7 and 6 .11, how
the factor axes for verbal and mathematical skills have moved
from outside the actual clusters (in the orthogonal solution)
to the clusters themselves (in the oblique solution).
Most factor-analysts work upon the assumption that correla-
tions may have causes and that factor axes may help us to identify
them. If the factor axes are themselves correlated, why not apply6*11 Thurstone's oblique simple structure axes for the same four men-
tal tests depicted in Figs. 6-6 and 6-7. Factor axes are no longer perpen-
dicular to each other. In this example, the factor axes coincide with the
peripheral vectors of the cluster.