Distributions of the relative difference between the computed equivalent width (W) and the true equivalent width (W_true) of Hβ lines with a 2 × 10⁴ K black body continuum for different assumptions on the line shape (narrow lines, Gaussian, Voigt, and Lorentz profiles) and different computation techniques. Each panel shows the result from 5 × 10⁵ random realisations of a spectrum for G = 12^m and ten transits. Positive values in the horizontal axis correspond to Hβ emission, negative values to Hβ absorption. For details see text in Sects. 8.3, 9.3, and 9.4.