Histogram shape parameter, ξ, as a function of log ₁₀(N_H/ cm^-2) in the Taurus region. The values are obtained using the τ₃₅₃ map at 10′ resolution and maps of the Stokes parameters Q_r and U_r that correspond to 1000 random-noise realizations. Each realization is generated using a Gaussian probability density function centred on the measured values Q and Uwith variances and . By joining the values at each N_H bin, we find a trend very close to the black line in Fig. 7, with little dispersion from the noise; much larger are the uncertainties in evaluating ξ at each N_H bin, as given in Fig. 7 but not shown here.