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st: performance of Hansen's J test of overid with number of instruments = N

From   Caterina Astarita <>
Subject   st: performance of Hansen's J test of overid with number of instruments = N
Date   Sun, 19 Sep 2010 11:35:22 +0100 (BST)

Dear all, 

I'm running a GMM system one step estimator with xtabond2 with 
a dataset where N = 19 and T = 15. I'm using the rule of thumb 
of taking the number of instruments less or equal to N. 
Furthermore I'm using the command collapse. With respect to 
Sargan and Hansen's J test of overidentification is 
respectively p = 0.502 and p = 1.00. The fact that I'm using a 
very low number of instruments is sufficient to rely on Hansen 
test? If I undestrand well the answer should be "no". 
According to Roodman "A note on the theme too many 
instruments"  in the  paragraph dedicated to "weak hansen test 
of instrument validity" is written that "a high p.value as, 
say 0.25 should be viewed with concerns". Thus I deduce that 
having a p = 1.00 is a bad signal and that a better scenario 
would be a p-value between 0.10 and 0.25 (or lower) . Is that 
correct? I know that is difficoult to establish a treshold but 
I would like to be certain I have understood Roodman's 

Thank you 


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