THE MISMEASL'RE OF MANThurstone reified his simple structure axes as primary mental
abilities and sought to specify their number. His opinion shifted as
he found new PMA's or condensed others, but his basic model
included seven PMA's—V for verbal comprehension, W for word
fluency, N for number (computational), S for spatial visualization,
M for associative memory, P for perceptual speed, and R for rea-
soning.*
But what had happened to g—Spearman's ineluctable, innate,
general intelligence—amidst all this rotation of axes? It had simply
disappeared. It had been rotated away; it was not there anymore
(Fig. 6.7). Thurstone studied the same data used by Spearman and
Burt to discover g. But now, instead of a hierarchy with a domi-
nant, innate, general intelligence and some subsidiary, trainable
group factors, the same data had yielded a set of independent and
equally important PMA's, with no hierarchy and no dominant gen-
eral factor. What psychological meaning could g claim if it repre-
sented but one possible rendering of information subject to
radically different, but mathematically equivalent, interpretations?
Thurstone wrote of his most famous empirical study (1938, p. vii):
So far in our work we have not found the general factor of Spearman.
... As far as we can determine at present, the tests that have been sup-
posed to be saturated with the general common factor divide their vari-
ance among primary factors that are not present in all the tests. We cannot
report any general common factor in the battery of 56 tests that have been
analyzed in the present study.
The egalitarian interpretation of PMA's
Group factors for specialized abilities have had an interesting
odyssey in the history of factor analysis. In Spearman's system they
were called "disturbers" of the tetrad equation, and were often
purposely eliminated by tossing out all but one test in a cluster—a
remarkable way of rendering a hypothesis impervious to disproof.
In a famous study, done specifically to discover whether or not
•Thurstone, like Burt, submitted many other sets of data to factor analysis. Burt,
chained to his hierarchical model, always found a dominant general factor and sub-
sidiary bipolars, whether he studied anatomical, parapsychological, or aesthetic
data. Thurstone, wedded to his model, always discovered independent primary fac-
tors. In 1950, for example, he submitted tests of temperament to factor analysis and
found primary factors, again seven in number. He named them activity, impulsive-
ness, emotional stability, sociability, athletic interest, ascendance, and reflectiveness.