The present edition of The Essentials of Mental Measurement contains four new chapters (reprints of recent papers by each of the authors), to indicate in some measure, however inadequately, what changes have taken place in the subject, and in their opinions, since 1925. So much has happened since then, in that province of experimental psychology to which the book refers, that a completely new work would be required to cover the ground; and in the circumstances of the present year such a new book is impossible.

A good many pages of the volume (especially chapters ix and x) are taken up with a critical discussion of Professor Spearman's Theory of Two Factors and Thomson's Sampling Theory. The present attitude of each author (Brown and Thomson) to this question can be gathered from the new chapters, and in Thomson's case from chapters in and xvm of his book The Factorial Analysis of Human Ability (London and Boston, 1939). Thomson has shown that the equations of the two theories can be transformed into one another, in either direction, by an orthogonal transformation. Brown has in co-operation with Stephenson assembled a large battery of tests of intelligence which conforms strictly (within sampling limits) to the requirements of the Theory of Two Factors. That theory has however itself undergone important developments and extensions in two directions. For the one direction, the last sentence in chapter xi, written in 1924, has proved to be prophetic.

“Our position is”, we then wrote, “that until the evidence is more clear we shall continue to suspect that numerous and wide group factors are present.” The presence of such group factors, in addition to g, is now universally recognised, and that mainly because of the work of Professor Spearman and his disciples in tracing and identifying them—although this school regards the number of general group factors as small. Indeed, there is now a school of thought, led by Thurstone, which has entirely dethroned g and replaced its “monarchic” rule by an “oligarchy” of group factors. In this present-day controversy we both find ourselves tending to prefer Spearman's rather than Thurstone's factors.