Sandra,
> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]] On Behalf Of
> Mortal, Sandra C.
> Sent: 25 January 2007 00:40
> To: [email protected]
> Subject: st: FW: Instrumental variables using two dummy
> variables as instruments
>
> Hi,
>
> I am running a regular OLS regression, but need to use
> instrumental variables because of an endogenous variable.
> This variable is continuous, but I am only able to think of
> two dummy variables to use as instruments (it is very
> difficult to find instruments for this variable). These seem
> to be good instruments because they are theoretically
> exogenous, and they have a strong correlation with the
> endogenous variable (F tests that these variables are equal
> to zero are well above 10). The problem is that in the
> second stage regression the coefficient of the instrumented
> variable is much higher than that of the endogenous variable
> in the plain OLS regression. I wonder if you could point me
> in the right direction. Specifically, are there problems
> with using dummy variables as instruments,
No, there aren't, or at least not in your context and nothing that you
haven't already thought about, e.g., the correlation with the endogenous
regressor.
> and do you know of
> a reason why the second stage regression coefficients are so
> high?
This may be perfectly reasonable, or it may be generated by invalid
instruments.
The sign of the OLS bias may be + or -, depending on the sign of the
correlation of the endogenous variable with the error term. If you are
using invalid instruments, they too can be biased, and again the
direction can be + or -. Thus, you can have b_IV > b_OLS because the
latter is biased downwards, the former is biased upwards, or both.
Wooldridge's u/g econometrics text, Introductory Econometrics, has a
clear discussion of this, though I don't have the page references handy.
By the way, since you have an overidentified equation, you should check
to see if it passes an overidentifying restrictions test.
Also by the way, you may want to use -ivreg2- for your estimation, since
it generates an overid stat automatically (as well as some other useful
diagnostics). Or you can use -overid- to get the overid stat.
> Is this normal?
> I am sorry this is not a directly STATA related question,
Last BTW - it's "Stata", not "STATA".
Cheers,
Mark
> but I have exhausted all of my other options, given my
> limited knowledge of statistics.
>
> Thank you,
> Sandra
>
>
>
>
> ***********************************************
> Sandra Mortal
> Assistant Professor of Finance
> 516 Cornell Hall
> University of Missouri
> Columbia, MO 65211
> Office: (573) 884-1684
> Fax: (573) 884-6296
> ***********************************************
>
>
>
>
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