That was so clear and helpful. Thank you so much Austin.
Best regards,
Nirina
On 9/25/07, Austin Nichols <[email protected]> wrote:
> Nirina F <[email protected]>:
> The IV estimate is always biased, but is less biased than OLS to the
> extent that identification is strong; in the limit of weak
> instruments, there would be no improvement over OLS in terms of bias
> and the bias would be 100% of OLS, and in the other limit, the bias
> would be zero percent. Clearly, you'd like to know where you are on
> that spectrum, even if only approximately. There is also a problem
> with the size of tests after IV--you think you are rejecting a
> hypothesis using a 5% alpha, but it is really 10% or 20%. Stock and
> Yogo did simulations to provide "rule of thumb" critical values (or
> rule of thumb "critical values" perhaps). Table 1 on page 39 of Stock
> and Yogo (http://ksghome.harvard.edu/~jstock/pdf/rfa_6.pdf) shows the
> value of the SY stat (a measure of the "strength of identification" or
> the predictive power of the excluded instruments) to limit the bias to
> 20% of OLS for two endogenous variables and three excluded instruments
> (n=2, K2=5) is 5.91 (similar values are given to limit the size of
> Wald tests in table 2, and stats for LIML estimates in tables 3 and
> 4).
>
> It's not clear what you don't understand from the Stock and Yogo
> paper, so it's hard to comment directly. If you gave your n and K2
> values and the SY stat from the output of -ivreg2- I suppose one could
> describe directly the situation in which you find yourself.
>
> But the key point is that all IV and IV-type specifications suffer
> from bias and size distortions, not to mention inefficiency and
> sometimes failures of exclusion restrictions. The SY stat gives you
> some measure of how strong your identification is in your sample, but
> no information about the validity of your instruments. Hope you also
> read BSS2007:
> http://econpapers.repec.org/paper/bocbocoec/667.htm
> and I expect -ranktest- will generate a new set of papers eventually
> (-ssc install ranktest- and -help ranktest- for more).
>
> On 9/25/07, Nirina F <[email protected]> wrote:
> > Dear all,
> >
> > Yes, I read
> > Stock, J.H. and Yogo, M. 2005. " Testing for Weak Instruments in
> > Linear IV Regression"
> >
> > but I still don't understand how do I interpret the critical values of
> > the Stock&Yogo- maximal IV size when using ivreg2.
> >
> > I think I compare the critical values from x%maximal IV relative bias
> > with the Cragg-Donald stat to test for weak instruments but the
> > critical values for the size with what and what it means?
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