Departure coefficients for some levels of Na, Mg, K and Ca obtained calculated in single-element mode (NLTE-s, in red) and in multiple-elements mode (NLTE-m, in black) for Procyon, the Sun and Arcturus. For each star, the same element abundances were used in all the LTE, NLTE-s and NLTE-m calculations. The fact that the b coefficient for the Mg II ground state increases toward the surface (due to NLTE effects in the Mg II h & k lines) inArcturus, but stays closely to 1 for the other two stars, follows from Mg II being the dominant stage of ionisation, and its ground state the most populated state, for Sun and Procyon, but not for the cooler starArcturus. The two grey vertical lines in each figure mark the depths in the atmospheres where τ_ross = 0.01 (left) and 1.0 (right).