1. A single asterisk, *, indicates significance at 10% level, a double asterisk, **, at 5% level and a triple asterisk, ***, at 1%; ? is the estimated autocorrelation parameter; obs. denotes the number of observations; NA: not applicable because the coefficients are not significant in the quadratic specification and the relationship appears to be increasing. AIC is the Akaike information criterion value; BIC is Schwarz's Bayesian information criterion; the RESET is the Ramsey specification test for omitted variables; BP is the Breusch-Pagan test for heteroskedasticity; BG is the Breusch-Godfrey LM test for the presence of first order autocorrelation;NOTES: Variables in natural logs. The regressions are estimated by GLS based on the PraisWinsten transformation