Correlations between estimated and true extinction in tests with stars simulated with the Besançon model. Results are shown for NICER (frame a)), Method B (frame b)), Method B with separate reference colours for four magnitude intervals (frame c)), Method T using a perfect extinction template (frame d)), or using the NICER map as the template with a single (frame e)) or four reference colours (frame f)). The final frames include bias correction with methods D1 and D2 (frames g) and h), respectively). Each frame lists the parameters of linear least squares, the bias Δ, and the rms error σ.