The effect of crossing times through the solar neighbourhood on the observed stellar age and metallicity distributions due to the oscillations perpendicular to the galactic plane is well known. Stars with high z-velocity are under-represented since they stay much longer far from the galactic plane than near the sun. Since the age and metallicity distributions of these high velocity stars are quite different from the age and metallicity distributions of all the stars moving at a mean distance 10 kpc from the galactic centre, a weighting procedure has to be adopted. The weight to be assigned to each star should be proportional to the ratio of the oscillation period in z, i.e. Tz, to the time spent within a sphere centered on the sun, i.e. τz. An approximate formula has been given by Woolley (1970): . The effect of such a weighting procedure is very important : the ratio Tz/Tz varies from 1 to more than A between young and old stellar populations. It has to be noted that the quadratic term of weighting (often neglected) is equal to the linear term for a |w| velocity of about 55 km/s.