Modulation index at 15 GHz versus the modulation index at the optical R band for the sample sources (HSPs red circles, ISPs blue rectangles). The sources for which one of the modulation indices could not be determined were excluded from the plot. As expected the ISPs in general have larger modulation indices in optical and radio than HSPs. Therefore the indices between the two band show significant (> 3σ) correlation (see text). The dotted line shows the best linear fit to the data.