consider a hypothetical system with continuously variable disorder (i.e., the parameters characterizing the disorder distribution such as the quenched charged impurity density and their strength as well as their spatial locations including possible spatial correlations in the impurity distribution can all be varied at will). Experimentally, however, the situation is qualitatively different. One does not typically change the impurity distribution in a sample in a controlled manner and make measurements as a function of disorder distribution. Experimentally, measurements are made in [*different*]{} samples and compared, and in such a situation there is no reason to expect two samples with identical mobility at some specified carrier density to have identical impurity distributions. The impurity distributions in different samples can be considered to be identical only if the full density-dependent mobility $\mu(n)$, or equivalently the density-dependent conductivity $\sigma(n)$, are identical in all the samples. Such a situation of course never happens in practice, and typically when experimental mobilities in different samples are quoted to be similar in magnitudes, one is referring to either the maximum mobility (occurring typically at different carrier densities in different samples) or the mobility at some fixed high carrier density (and not over a whole range of carrier 