Median values of the difference between observed and true metallicity as a function of true metallicity values at z = 0.30 and S /N_I,obs = 10. The black error bars show results for our analysis including all spectral indices, while the red dashed error bars represent the case using only the 5 absorption indices used in Gallazzi et al. (2005). The blue vertical dashed line marks the solar metallicity value ([Z/H] = 0). The green dashed line represents the 0 value of the y-axis.