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 <[email protected]>
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
*
* For searches and help try:
* http://www.stata.com/support/faqs/res/findit.html
* http://www.stata.com/support/statalist/faq
* http://www.ats.ucla.edu/stat/stata/
