We present a theoretical study on differential group-delay (DGD) statistics for common models of polarization-mode dispersion (PMD) emulators. Our study will show, for the first time to our knowledge, that the statistics for the length of the PMD vector will not necessarily behave as a system with three degrees of freedom when the number of sections in the PMD emulator is low. However, when the number of sections is Large, i,e,, 10 sections or more, the length of the PMD vector is well described with a system with three degrees of freedom.