Example cross-correlation of magnification histograms. The colored lines describe the probability of observing a magnification difference of Δm between images a and b of the B 0128+437 lens system for the model with N_bh = 300 and = 5 pc. The dashed line represents the actually observed difference. We note that the smoothing of Δm = 0.24 to account for observational or model errors has already been applied to these histograms.