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The measurement of psychological variables poses many novel and difficult problems. It is somewhat odd that in spite of extensive discussions of scientific method, measurement should occupy such a minor role. Some theoreticians ignore it altogether. It is the purpose of this paper to propose a systematic approach to the whole subject.
One of the most startling discoveries in modern physics is the measurement paradox, usually referred to as the principle of uncertainty or indeterminacy. While the exact interpretation of this principle in quantum mechanics is a matter of dispute 17 it is clear that in all physical measurement, the interaction between observer and object measured is a real factor that must be given due consideration. In macroscopic physics it can in many cases be ignored (though its theoretical existence is admitted); in subatomic physics it cannot, and the complexities involved in measurement become the jumping-off point for an altered view of the physical world. 21
An eminent physicist, Niels Bohr, has commented on the parallel between the situations in physics and psychology. In his book, Atomic Physics and Human Knowledge 2 he says:
Indeed, the necessity of considering the interaction between the measuring instruments and the object under investigation in atomic mechanics exhibits a close analogy to the peculiar difficulties in psychological analysis arising from the fact that the mental content is invariably altered when the attention is concentrated on any special feature of it. (p. 11.)