This is a book about an error, an error in scientific method fundamental to quantitative psychology. This error became locked into established ways of doing things in that science, that is, it became systemic. Then it was compounded by a higher order error, the effect of which was to disguise the first. Because science is a cognitive enterprise, because scientific methods are fallible methods, and because all scientists are fallible cognisers, the making of errors is par for the course in science and so any particular instance of error is usually only of passing interest. In so far as scientists invite criticism and put their ideas to the test, there is some chance that errors will eventually be corrected. On the other hand, errors that become systemic are of more than passing interest because they show that science's mechanisms for correcting error are themselves fallible and able to break down. Then it is of interest to inquire into the conditions of such errors because they may teach us something about the workings of science. This book is written as a contribution to that endeavour.

In the case studied here, the first of the two errors mentioned was of a familiar enough kind. It was the error of presuming an answer to a scientific question, rather than investigating it empirically. Quantitative psychologists presumed that the psychological attributes which they aspired to measure were quantitative. There is no question that presuming instead of testing was an error in scientific method.