The relationship between the mean metallicity and the distance from the galactic plane. The straight lines are the mean-square regressions, the bars are the errrors of the mean. The slopes of the lines determined the gradient values which are equal from top to bottom to: -(0.17±0.02) kpc-1 -(0.15±0.02) kpc-1 -(0.12±0.06) kpc-1 and -(0.11±0.09) kpc-1.discussion in the text | previous figure