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Histograms of phase-difference data for ordinary- and extraordinary-mode signals reflected from the D region of the ionosphere are compared with two phenomenological scattering models: (a) a small-scale homogeneous turbulence model and (b) a single reflecting-layer model. Although the turbulence model fits the data somewhat better than the single reflecting-layer model does, the predictions of both models are in surprisingly close agreement on virtually all aspects of differential-phase data. In particular, both models are shown to predict a broadening of the histograms with increasing altitude as found experimentally. This is attributed to the increasing difference between the differential phases of signals scattered from the top and bottom of the scattering volume. Thus, by operating at a higher frequency or using a shorter or a coded pulse, one should obtain useful differential-phase data at higher altitudes.