Using a large, unbiased sample of 374 optically selected QSOs, of which 54 are radio detected, we have computed for different redshift ranges the probability distribution function φ (R) of R defined as the ratio of monochromatic luminosities at 15 GHz and 2500 Å in the QSO's rest frame. A significant variation of φ (R) with redshift is noticed. At small redshifts (z < 0.5), the distinctive feature of φ (R) is a peak near R∼102, arising due to the QSOs having steep radio spectrum(αr<−0.5).