# st: RE: statalist-digest V4 #970

 From "Paul Seed" To Subject st: RE: statalist-digest V4 #970 Date Thu, 8 Aug 2002 13:42:51 +0100

```Many thanks to Gary for this.  Changing the saved result to a scalar
did indeed take care of the problem.

Although the term "control" is used, this is not a matched case-control
study, rather a comparison of two groups of pregnant women, (with & without
APS).  An unpaied t-test (or non-parametric equivalent) is entirely
appropriate.

Date: Wed, 07 Aug 2002 10:47:41 -0700
From: Gary Longton <glongton@fhcrc.org>
Subject: Re: st: Problems with -bs- command

Paul Seed wrote:

> I am interested in a bootstrapped confidence interval for the
> difference between two means, and to save time
> wish to work out the means using -summ, meanonly-.
>
> I have written a Stata program to do this, which runs perfectly well
> on it's own.  However, when trying to get -bs- to work out the confidence
> intervals, I get an unexplained error message.
>
> Any thoughts on wher ethe problem lies, greatly appreciated.

the program:

> cap prog drop myprog
> prog define myprog, rclass
> 	summ centile if aps, mean
> 	di "r(mean) = " r(mean)
> 	local m1 = r(mean)
> 	return local m1 = r(mean)
> 	summ centile if control, mean
> 	return local m2 = r(mean)
> 	return local m2_m1 = `m1' - r(mean)
> end myprog

called with -bs-

> 	set seed 35435141
> 	bs "myprog" "r(m1)", reps(50)

or

> 	bs "myprog" "r(m1) r(m2) r(m2_m1)", reps(50)

and output:

> . * Now use it for bootstrap"
> .         set seed 35435141
>
> .         bs "myprog" "r(m1) r(m2) r(m2_m1)", reps(50)
>
> command:     myprog
> statistics:  r(m1) r(m2) r(m2_m1)
> error in statistic: r(m1)
> r(198);

I believe the -bs- command is complaining about the use of macro results
placed
in the expression list, i.e. "r(m1)"

It should work if these are instead scalar results.  Eg. in the program
- -myprog-, replace

return local m1 = r(mean)

with

return scalar m1 = r(mean)
^^^^^^

and likewise for m2, and m2_m1
and it should work.

However, am wondering whether this type of sampling is best for your design?
The command

> 	summ centile if control, mean

hints that this might be a case-control design?, in which case you might
want to
be sampling separately from cases and controls? which can be done, but not
with
- -bs-.

- - Gary

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```