The quantum mechanical interference method to extract oxide thickness is examined in detail by fully using the least square fit technique. The current density, J(o), that corresponds to oscillatory peaks of n = 1, 2, 3 and 4 is plotted against n. and by doing so, data scattering among the devices is clearly recognized. The extracted physical parameters. Phi (M) and m(2), have large standard deviation around the average depending on del ice and polarity conditions. Despite the scattering, an apparent success of the quantum interference (QI) analysis is obtained when the averaged parameters, Phi (M) = 2.94 eV and m(2)/m(o) = 0.54, are used. It is discussed that both Phi (M) and m(2) are equally critical to the accuracy of the QI method, and they luckily deviate in the opposite direction around the average to cancel with each other. An essential limitation of the QI method is that the device characteristics depend on the factor (t(OX)root Phi (M)) and we cannot decide Phi (M) and t(OX) separately. t(OX) to extract Phi (M) and m(2) must be evaluated by the help of other technique like ellipsometry or C-V measurements.