Error message GMM estimation

[anna mantelli]

Hi everyone,
I need to perform a GMM estimation, but the Stata output shows an error message 'could not evaluate equation 1'. This is the model I'm attempting to estimate:

gmm (D_LCO2 - {b0}*L_D_LCO2 - {b1}*L_D_LP - {b2}*L_D_LA - {b3}*L_D_LIND - {b4}*L_D_LS - {b5}*L_D_U - {b6}*L_D_LPD - {b7}*L_e2), instruments(L_D_LCO2 L2_D_LCO2 L2_D_LP L2_D_LA L2_D_LIND L2_D_LS L2_D_LU L2_D_LPD L2_e2) onestep

Here the variable description:
D_LCO2: first difference of the logarithm of CO2
L_D_LCO2: first difference of the logarithm of CO2 lagged once
L_D_LP: first difference of the logarithm of P lagged once
L_D_LA: first difference of the logarithm of A lagged once
L_D_LIND: first difference of the logarithm of IND lagged once
L_D_LS: first difference of the logarithm of S lagged once
L_D_LU: first difference of the logarithm of U lagged once
L_D_LPD: first difference of the logarithm of PD lagged once
L_e2: e2 lagged once
L2_D_LP: first difference of the logarithm of P lagged twice
L2_D_LA: first difference of the logarithm of A lagged twice
L2_D_LIND: first difference of the logarithm of IND lagged twice
L2_D_LS: first difference of the logarithm of S lagged twice
L2_D_LU: first difference of the logarithm of U lagged twice
L2_D_LPD: first difference of the logarithm of PD lagged twice
L2_e2: e2 lagged twice



Do you have any idea about what might be causing this error?

Thanks