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Since Horst (1941) initially introduced the concept, a number of investigators have suggested alternative definitions that both include a broader class of situations and are more precise. Following a conceptual framework suggested by Conger (1974), the present paper discusses a definition based on the relation of the semipartial correlation to the zero order correlation. This definition possesses a number of advantages, including the fact that it can be easily extended from the two predictor case to the general p-predictor case.
Wayne F. Velicer (Fri,) studied this question.