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st: RE: Minimum number of observations per cluster for clustered standard errors in IV?

From   "Schaffer, Mark E" <>
To   "" <>
Subject   st: RE: Minimum number of observations per cluster for clustered standard errors in IV?
Date   Thu, 17 Oct 2013 15:23:38 +0000


My understanding from Austin's work (BTW, you cite a 2007 paper by him - the Statalist FAQ asks you to provide a full reference in such cases) is that problems arise with unbalanced clusters (=some clusters have few obs, some have lots), not small clusters (=clusters with few obs) per se.

The easy way to see that small clusters per se are not necessarily a problem is by noting that the classic Eicker-Huber-White-sandwich-heteroskedasticity-robust covariance estimator is just a special case of the cluster-robust covariance estimator with 1 observation per cluster.


> -----Original Message-----
> From: [mailto:owner-
>] On Behalf Of Jen Zhen
> Sent: 17 October 2013 14:38
> To:
> Subject: st: Minimum number of observations per cluster for clustered standard
> errors in IV?
> Dear listers,
> I sent this question as an add-on to my earlier inquiry under a different
> headline and just realized that this way it may not reach its audience, hence I'm
> putting it into a different thread now:
> I am trying to choose how to cluster my SEs in ivreg2. I understand from Nichols
> (2007) and Angrist and Pischke that I should pick a dimension with which I get
> at least 42 or 50 different clusters. Now that's in extremo easy to satisfy if I
> cluster by observation. But I guess there should also not be too few
> observations per cluster. But I haven't found anything on what is "too few"
> here.
> Is there any evidence providing for a minimum number of obs per cluster, or a
> minimum on the average, median, P10 etc number? Any other useful criterion?
> I suppose that when using 2 way clustering then each of my 2 dimensions
> should on their own satisfy the respective requirement?
> Thank you so much and kind regards,
> JZ
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