Best-fit slope and intercept in the 3–10 keV band for the rev sub-samples (filled-blue circles) and the VD sub-samples (open-red squares) as a function of the average duration of their sources. The circles indicate the first rev and VD sub-samples (i.e those with a shorter duration) for which the best-fit results are in agreement (within errors). The horizontal lines indicates the best fit value when we fit the combined data from the two sub-samples in agreement (rev and VD).