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