Re: st: Bootstrap command when used with cluster and strata options

[jpitblado@stata.com (Jeff Pitblado, StataCorp LP)]

Chris Frost <Chris.Frost@lshtm.ac.uk> is using -bootstrap- with options
-strata()-, -cluster()- and -idcluster()-, and noticed that the new cluster
variable repeates ID values (starting from 1) between the strata:

> I think that there is a problem with the bootstrap command when used in
> conjunction with the "cluster" and "strata" options. The problem arises
> because the command "bootstrap, strata(group) cluster(id) idcluster(newid)
> ....." creates a variable "newid" which is only unique (at the cluster
> level) within each strata. For example if there are 1000 subjects (with
> multiple measures per subject) each with a unique id but in two equal size
> groups the above command will result in each bootstrap sample having only
> 500 values of newid with subjects being erroneously paired up: this will
> lead to incorrect variance estimates with a command such as
>
> . bootstrap, strata(group) cluster(id) idcluster(newid):
>		 mixed outcome i.group || newid: 
> 
> Am I correct? Can this be fixed?

Austin Nichols <austinnichols@gmail.com> verified this, and pointed out that
-bsample- is the command that is producing the new cluster id variable.

Jeph Herrin <stata@spandrel.net> ran across this behavior in a reply to a
Statalist thread earlier this year.  Sorry I missed that thread Jeph.

The documentation for -idcluster()- for -bsample- says:

	idcluster(newvar) creates a new variable containing a unique
		identifier for each resampled cluster.

This description agrees with Chris's expectation.  As such, we will update
-bsample- to behave as expected.

--Jeff
jpitblado@stata.com
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