Synthetic stellar population to simulate the selection effect of Gaia-ESO survey and of the observed stellar sample. Here, the results of the simulation at 1 kpc are shown for the blue box (top row) and red box (bottom row), as defined in Sect. 2. Right: completeness fraction, defined as the ratio of stars with cut to the number of stars without cut, that is, the larger the number the more stars are retained in the population after applying the colour cuts (Sect. 2). In case of no bias, the completeness fraction is unity 1.