[Gretl-users] a problem about panel data

hi, when I replicate Example 11.3 of Wooldridge's advance book(the second edition), i encounter a problem. To run the pooled OLS, I write open airfare.dta genr lfare_1=lfare(-1) diff lare lfare_1 concen ols d_lfare d_lfare_1 d_concen then, I get the outputs, which is different from the results of the Book. Why? Thanks! Model 1: OLS, using observations 3-4596 (n = 4594) Dependent variable: d_lfare Heteroskedasticity-robust standard errors, variant HC1 coefficient std. error t-ratio p-value ------------------------------------------------------------ const 7.61258e-05 0.00427816 0.01779 0.9858 d_lfare_1 -0.114781 0.0104885 -10.94 1.56e-27 *** d_concen -0.292149 0.0520106 -5.617 2.06e-08 *** Mean dependent var 0.000068 S.D. dependent var 0.294573 Sum squared resid 386.0532 S.E. of regression 0.289981 R-squared 0.031358 Adjusted R-squared 0.030936 F(2, 4591) 70.98109 P-value(F) 4.37e-31 Log-likelihood -830.0114 Akaike criterion 1666.023 Schwarz criterion 1685.320 Hannan-Quinn 1672.815