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Re: st: Critical values for stock and yogo test when the clusteroption is used
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From |
"Danny Cohen-Zada" <[email protected]> |
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To |
<[email protected]> |
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Subject |
Re: st: Critical values for stock and yogo test when the clusteroption is used |
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Date |
Wed, 27 Feb 2008 19:22:47 +0200 |
Dear Professor Shaffer,
I thought again and i think i understood that i should compare 5.26 to 16.38
and i should be worry about the strength of my instrument. Am i right?
Danny
----- Original Message -----
From: "Schaffer, Mark E" <[email protected]>
To: <[email protected]>
Sent: Wednesday, February 27, 2008 5:18 PM
Subject: RE: st: Critical values for stock and yogo test when the
clusteroption is used
Danny,
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of
Danny Cohen-Zada
Sent: Wednesday, February 27, 2008 2:58 PM
To: [email protected]
Subject: Re: st: Critical values for stock and yogo test when
the clusteroption is used
Dear Professor Austin and other statalist subscribers
I only want to add that in the email today I clarified
something that wasn't clear enough in the private mail
yesterday. The points is that the results much differ if I
use the clustered f-statistic instead of the regular one.
The regular f-statistic is 158 which is extremely above the
regular critical value (the critical values are between
5.53-16.38). This indicate that the instrument is extremely
strong. However, if i compare the clustered f-statistic
(5.26) to the regular critical values I may come to an
opposite conclusion that my instrument is quite weak (it is
only close to the 25% maximal iv size). Should i worry about
the strength of my instrument?
The short answer is "yes", you should worry, and your intution below is
right.
The problem starts out just like the usual problem of using non-robust
SEs for inference. If the disturbance is heteroskedastic or clustered,
the usual SEs are wrong, and usually [sic] you'll get SEs that are "too
small" and test stats that are "too big". The same features of the data
that give you these test stats that are "too big" will give you a
first-stage F-stat that is also "too big", and so the Stock-Yogo
critical values will be wrong.
Where it gets complicated is that the S-Y critical values for weak
identification come from Monte Carlos. My understanding is that if you
want the "right" critical values for cases where the standard S-Y iid
assumption is loosened, you have to specify how it's loosened, i.e., in
your case, what kind of clustering you've got.
That said, using a cluster-robust first-stage F stat with the S-Y
critical values is not bad and in the absence of anything better is
probably the best you can do.
It's also worth noting that the first-stage F-stat can also be used as a
test for *under*identification a la Anderson. (See the paper by myself,
Kit Baum and Steve Stillman in the latest issue of the SJ, vol. 7 no. 4
2007.) This is a test with an asymptotic justification and uses
standard critical values, and the robust first-stage F stat with this
standard critical values is fine. Since underidentification is a lower
hurdle than weak identification, if you can't reject the null that your
equation is underidentified, you can pretty safely also fail to reject
the null that it's weakly identified.
Hope this helps.
Cheers,
Mark
Prof. Mark E. Schaffer
Director
Centre for Economic Reform and Transformation
Department of Economics
School of Management & Languages
Heriot-Watt University
Edinburgh EH14 4AS UK
44-131-451-3494 direct
44-131-451-3296 fax
http://www.sml.hw.ac.uk/cert
After all, i guess that as the
clustered f-statistic is much lower than the regular one, the
clustered critical value should also be lower? That is, don't
we require to much when we compare the clustered f-statistic
to the regular critical values.
Best,
Danny
----- Original Message -----
From: "Austin Nichols" <[email protected]>
To: <[email protected]>
Sent: Wednesday, February 27, 2008 3:36 PM
Subject: Re: st: Critical values for stock and yogo test when
the clusteroption is used
> All--
> My reply (Tue, Feb 26, 2008 at 4:42 PM) to this
interlocutor's private
> email yesterday was:
> ***************
> Until someone redoes Stock and Yogo with various kinds of
clustering,
> you are stuck comparing your value to their table. One
reason no one
> has done that is that different kinds of clustering might produce
> different answers, so the critical values might be more
> problem-specific, and you might have to run your own
simulations for
> your own case, using observed intra-cluster correlations in
the data.
> ***************
> If anyone has a different answer, I'd be interested to hear
it. Note
> that the ref for Stock and Yogo is in the help for -ivreg2-
on SSC and
> in http://www.stata.com/meeting/5nasug/wiv.pdf.
>
> On Wed, Feb 27, 2008 at 7:33 AM, Danny Cohen-Zada
<[email protected]>
> wrote:
>> I would be thankful to anybody that can give me an advice
concerning
>> the critical values in the stock and yogo test.
>>
>>
>> I estimate the following IV model with the cluster option
>>
>> ivreg2 y1 x1 x2 (y2 = z1 ), ffirst cluster (x3)
>>
>> (In my model I have only one endogenous regressor and one
excluded
>> instrument).
>>
>> I know that the Stock and Yogo statistic assume conditional
>> homoskedasticity and independence. Since I use the
cluster option I
>> can not use the reported stock and yogo statistic (f statistic on
>> the exluded instrument) and the reported critical values.
>>
>> I know that I can use the command ffirst to obtain the
corrected f
>> statistic on the excluded instrument. It differ
substantially from
>> the unclustered f statistic (regular = 158, clustered=5.26, the
>> regular critical values are in the range 5.53-16.38). However, I
>> still have a problem because I do not know what the corrected
>> critical values are (I guess that I can not use the
regular critical
>> values). If i used the regular f-statistic and the regular
critical
>> values i find that my instruments are very very strong.
This picture
>> is changed if i compare the corrected f-statisticto the regular
>> critical values.
>>
>> I would be thankful to anybody who knows how I can check that my
>> instruments are not weak in a regression where the
cluster option is
>> used.
>> Specifically,
>> to which value should I compare the corrected f-statistic on the
>> excluded instrument obtained by the command ffirst.
>>
>>
>> Best,
>>
>> Danny
>>
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