Absolute ratio of the maximum flux density of a Stokes V pixel, S _V,p, to the total flux density of the corresponding Stokes I source, S_I, as a function of signal-to-noise ratio, S_I/σ_I, in Stokes I. The contours associated with the density plot correspond to 300, 900, 2500, 5000, 10 000, 20 000, 30 000, 40 000, and 50 000 measurements. The colour bar at the top-right-hand corner indicates the number of counts in the density plot. The red line is the trend measured when the aperture used to calculate the maximum Stokes V signal is replaced with Gaussian random noise. The simulation follows the measured data well up to S_I/σ_I ~ 10³, the point at which leakage becomes dominant. The histogram in the right panel shows the distribution of |S_V,p/S_I|, which is dominated by low signal-to-noise sources.