Ratios of 10⁵ realizations of Eqs. (6) and (7). The interpolation is performed with a Lanczos kernel for random locations inside the inner ±3σ of a Gaussian emission line with a FWHM of 3.5 pixels (assuming Poisson noise). The pixel responses are modeled with a normal distribution with μ = 1 and σ = 0.02. In both panels (top: flux ratios, bottom: error ratios) the solid red line shows the ensemble mean, the dashed red line its standard deviation, and the blue dashed line the standard deviation when the data are rectified but flat fielding is not applied afterwards.