Dear Stata-listers,
I am trying to estimate the following using ivreg2
xi: ivreg2 lriceyield2 (counting = per3 per4 per5 per6 per7 per8 per3n per5n per6n per7n) birth_north2 exper2 expersq2 aland2 anugood2 anubad2 ianulandsha2 i.comm92 if d92==1 & urban92==0, first rob gmm
and obtain the following message:
First-stage regressions
Warning: collinearities detected among instruments
1st stage tests of excluded exogenous variables may be incorrect
[output omitted]
As I was trying to figure out what this meant, I tried to the following : to drop all excluded IVs except one which I know is the strongest one: per3n.
xi: ivreg2 lriceyield2 (counting = per3n ) birth_north2 exper2 expersq2 aland2 anugood2 anubad2 ianulandsha2 i.comm92 if d92==1 & urban92==0, first gmm
And I obtain the same warning message.
Would someone know what is happening?
Thanks a lot in advance,
Thi Minh
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