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We present the theoretical framework that provides for a generalization of the standard interferometric method of polarization-mode dispersion (PMD) measurement. The basic formula linking the root-mean-square (rms) differential group delay (DGD) to the interferograms is derived independently of previous assumptions regarding the coupling regime, the shape of the interferograms, or the width and smoothness of the source spectrum. Moreover, the measured rms-DGD value is shown to be a well-defined weighted average over a finite wavelength range that can be rigorously traced to DGD measurements. Numerical simulations confirm the mathematical exactness of the analysis, and experimental tests confirm the extended range of validity provided in practice by the generalized method.