Comparison between two different methods for estimating the significance level of absorption line features. The x-axis corresponds to the log (〈S/N〉_res × N/b) values proposed by Richter et al. (2006) (at least for broad H i lines) where N is in units of cm⁻² and b in km s⁻¹, while the y-axis corresponds to our W_r/δW_r (see Section 4.3). Blue triangles correspond to narrow H i absorption components (b < 50 km s⁻¹), while red pentagons correspond to broad H i absorption components (b ≥ 50 km s⁻¹), both found across the full Q1410 spectrum. White crosses mark components having multiple transitions, hence reliable (label ‘a’; see Section 4.5). The grey shaded area correspond to values W_r/δW_r < 3, while the vertical dashed line correspond to a value of 〈S/N〉_res × N/b = 2.5 × 10¹² cm⁻² (km s⁻¹)⁻¹. See Appendix B for further details.