In the perturbative QCD analyses of the deeply inelastic processes, the coupling constant depends on at least two mass· scales, the renormalization scale and the factorization scale. By integrating the coupled renormalization group equations with respect to these two mass·scales, the running coupling constant is defined. A perturbative approximation then introduces a new ambiguity, the integration· path dependence, into the theory. We show that the problem of this new ambiguity is resolved by

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... posing Stevenson's principle of minimal sensitivity. Together with the analogous analysis ofthe operator matrix element or the cut vertex, we can completely solve the problem of getting an unambiguous perturbative QCD prediction.