Various fit of the reference numerical BSD (gray histogram) using power law parameterization from Sect. 3.1.1 (green lines) or logarithmic Softplus parameterization from Sect. 3.1.2 (orange lines) considering the dispersion (dashed lines) and not (solid lines). A numerical fit of the reference BSD using Eq. (1) from Furlanetto et al. (2004) inferring the efficiency parameter ζ is shown as a comparison (yellow dashed line). The fit is done at x_H II ∼ 1% (left panel) and x_H II ∼ 3% (right panel).