st: RE: OLS and IV have opposite sign

[Sun Yutao <yutao.sun.statalist@outlook.com>]

Hello Shikha,

I don’t know the exact situation you are in but, 

If one or more variables are correlated with the error term, the OLS is biased and when you are correcting that by IVs, the estimates of course will be different.

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Shikha Sinha
Sent: Monday, October 15, 2012 9:58 PM
To: statalist@hsphsun2.harvard.edu
Subject: st: OLS and IV have opposite sign

Dear all,

I am estimating an Ordinary least square (OLS) and Instrument variable
(IV) model, however the signs are opposite to each other. The OLS coeff is negative, while the IV coeff is positive. Could anyone explain what the signs in these two models are different- is there any econometric reason for this?

Thanks,
Shikha
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