However,the use of classical linear regression assumes that the regression residuals are independent, while the data in this study were correlated. Indeed, the residuals autocorrelation test of Durbin–Watson shows that in most cases the observed value of DW statistic is lessthan the critical value of the lower bound (1.201) read in the table of Durbin–Watson for a sample size equal to 20 and one explanatory variable at the 5% level