Upper right: uncertainty distribution owing to the rms of the H i profiles, ΔA(rms). Upper left: uncertainty distribution produced by errors in the mean velocity, ΔA(mean   vel). Bottom: uncertainty distribution of A_{flux ratio} for the H i sample (N = 312) combining the effect of ΔA(rms), ΔA(mean   vel) and the small contribution of ΔA (pointing offset) (See Sect. 3.2.1). The H i refined subsample (ΔA_{flux ratio} < 0.05, indicated as a dotted line in the lower panel) is shown in the plots as blue filled histograms. Best Gaussian fits are presented for all distributions as dashed lines: for the H i sample: a) μ = 0.02, σ = 0.02; b) μ = 0.04, σ = 0.02; and c) μ = 0.04, σ = 0.03; and for the H i refined subsample: a) μ = 0.012, σ = 0.007; b) μ = 0.029, σ = 0.012; and c) μ = 0.033, σ = 0.012.