thus it can be used as a reliable estimate of the integrated SFR. Moreover another advantage of using $\rm H\alpha$ E.W. over $\rm \Sigma~H\alpha$ is that the accuracy of the former is about 10%, while the uncertainty on the area determination, up to 100%, reflects into a similar uncertainty on $\rm \Sigma~H\alpha$.

The Frequency Distribution of the $\rm H\alpha$ E.W.
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The analysis carried out in the previous section has shown that the SFR in disk galaxies is anti-correlated with their luminosity (mass). This finding makes the determination of the characteristic SFR properties of galaxies meaningless, unless a luminosity range is specified, i.e. if samples selected according to well defined completeness criteria are used. The $\rm H\alpha$ survey presented in this paper contains in fact a subsample complete to $\rm M_p = 
-18.8$, being selected from a magnitude limited subsample complete down to $\rm m_p=15.4$, entirely composed of objects lying at a constant distance (70 Mpc) (containing both isolated and cluster members). With these data we construct the frequency distribution of the $\rm H\alpha$ E.W. in bins of Log E.W.=0.4, as given in Fig. 9a. It appears that, for galaxies brighter than $\rm M_p = 
-18.8$, the average $\rm H\alpha$ 