
Dear,
I built the generalized method of moments model to estimate the sales rank
in the bookstore using plm package in R.
 
The equation is:
 
data1.gmm <- pgmm(dynformula(lnsales_rank ~ ln_price + avg_ham_rate +
avg_spam_rate + num_of_ham+ num_of_spam + ship_code2 +ship_code3
+ship_code4+ ship_code5+ ship_code6 + ship_ code7, lag = list(0, 0, 0,
0,0,0,0,0,0,0,0,0), log =FALSE), data=data, effect = "individual", model =
"twosteps", transformation = "ld",gmm.inst = ~ ln_price, lag.gmm =2)
 
However, error message: Error in solve.default(Reduce("+", A1)) :
  system is computationally singular: reciprocal condition number =
6.36689e-23" shown.
 
I found that skipping one of the ship_code variables (dummy variable) can
generate result. But don't know the reason. 
 
I found some books about using R, but no one can answer me.
 
Please help. Thanks much. 
 
Zoe 		 	   		  
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