We examine the impact of discrete numbers of stars in stellar populations on the results of Chemical Evolution Models .
We explore the resulting dispersion in the true yields and their possible relation with the dispersion in observational data based on a Simple Closed-Box model . In this framework we find that the dispersion is larger for the less evolved or low abundance regions .
Thus , the age-metallicity relation may be a tracer of the Star Formation History of our Galaxy .
This theoretical dispersion is especially high for the relative abundance log ( N/O ) in regions where the total number of stars created is still low .
This may explain part of the scatter in the N/O ratio observed in star forming galaxies . We have also found a first order theoretical estimation for the goodness of a linear fit of the Helium abundance vs. 12 + log ( O/H ) with values of the regression coefficient between 0.9 and 0.7 ( independent of sampling effects ) . We conclude that it is necessary to include these sampling effects in a more realistic Chemical Evolution Model in order that such a model reproduces , at the same time , the mean value and the dispersion of observed abundances .