PREFACE

DURING the past ten years, psychologists, both in this
country and in America, have shown a rapidly increasing
interest in factor-analysis as an instrument of research.
In Great Britain, following the remarkable lead of Professor
Spearman over thirty-five years ago, a vast stream of
factorial work has issued from our laboratories and schools ;
but a large proportion of it—particularly that which deals,
not with results, but with methods—has remained buried in
postgraduate theses or in special reports, and so has never
become generally known. Indeed, until quite recently,
factor-analysis has been looked upon as a peculiar and some-
what isolated branch of psychology—at best a field for
specialists, at worst the dubious hobby of an esoteric school,
but in any case beyond the ken of the ordinary scientific
reader.

Nevertheless, though its abstract foundations are still
the subject of some controversy, its concrete applications,
particularly in the spheres of education and vocational
guidance, have proved more stimulating and more fruitful
than any other line of approach. " The entire practice of
mental testing and the whole body of individual psychology
rest," we have been told, " upon a factorial basis." To-day
the theory of mental factors is discussed in almost every
psychological textbook. All our students are expected to
know something of its leading principles and of its main
achievements. And yet there is no work of reference on
the subject that does not presuppose a mathematical back-
ground that very few students possess. The majority see
in it their fons asinorum : for factor-analysis is still presented
to them as an abstruse statistical technique, too specialized
for any but the advanced mathematician to follow or
employ.

The standpoint advocated in this book is very different.