Study of the recovery of the properties of the ionised gas for observations with the medium resolution grism (V600). Estimated relative random errors in the measurements of the Hα/Hβ ratio as a function of the equivalent width of Hβ emission lines (EW_Hβ) for mock galaxies with an old (left panels) and a young (right panels) underlying stellar population and different values for the dust attenuation: magenta for A_V = 0.1, red for A_V = 0.3, green for A_V = 0.6, and blue for A_V = 1.0. Top panels are for dust attenuation fixed in the program, and bottom panels for dust attenuation fitted in the program. Each block of the histogram represents an error of 5%.