Comparison of observed and modelled H i line decrements. The ratios are normalized to H i (6–5) for a sample of Class II sources and overlaid with model predictions from KF11 at a fixed temperature of 8750 K. Each panel corresponds to an individual source, with blue-filled circles representing the observed line ratios and downward red arrows denoting upper limits for non-detections. Coloured model curves indicate the predicted line ratios for varying hydrogen densities shown in the colour bar above.