Rest-frame Si IV time lags as a function of C IV lags. All the available data from the literature are shown, together with our measurements from the RM of APM 08279+5255 (legend is on the plot). We performed a free linear fit in the logarithmic space (solid line) and a linear fit with logarithmic slope fixed to unity (dotted line) for the whole data set. Fit coefficients are indicated in the plot with their 1σ errors.