**3. Conclusions**

The following major conclusions were drawn from the work. A satisfactory theoretical model was obtained to explain the observed D–P track, with sources at z ≥ 1 showing stronger anti-correlation than those around z < 1. A problem was developed for these objects to match-up with D–z positive correlation as predicted in this paper at z < 1. Finally, the distributions of radio galaxies and quasars on the *x*–D plane are consistent with the beaming scenario, with quasars being more consistent with relativistic beaming and source orientation model than radio galaxies.
