Thanks to Samuel for the promotion. However, -cendif- computes confidence intervals, not for differences between percentiles, but for percentile differences. These are not exactly the same thing. For a counterexample, see Newson (2008).
If Suzanne wants confidence intervals for the differences between 2 5th percentiles for 2 subpopulations, then Suzanne should probably use the -centile- command to get confidence intervals for the 2 5th percentiles, and then use the -invcise- package and the -metaparm- module of the -parmest- package, downloadable from SSC, to get a confidence interval for the difference between the 2 5th percentiles. The -invcise- package calculates standard errors using the inverse confidence interval method.
I hope this helps.
Best wishes
Roger
References
Newson, R. B. Hodges-Lehmann median differences between exponential subpopulations. Downloadable from
http://www.imperial.ac.uk/nhli/r.newson/papers.htm#miscellaneous_documents
as from 12 October 2008.
Roger B Newson BSc MSc DPhil
Lecturer in Medical Statistics
Respiratory Epidemiology and Public Health Group
National Heart and Lung Institute
Imperial College London
Royal Brompton Campus
Room 33, Emmanuel Kaye Building
1B Manresa Road
London SW3 6LR
UNITED KINGDOM
Tel: +44 (0)20 7352 8121 ext 3381
Fax: +44 (0)20 7351 8322
Email: [email protected]
Web page: http://www.imperial.ac.uk/nhli/r.newson/
Departmental Web page:
http://www1.imperial.ac.uk/medicine/about/divisions/nhli/respiration/popgenetics/reph/
Opinions expressed are those of the author, not of the institution.
-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of [email protected]
Sent: 10 June 2009 22:42
To: [email protected]
Subject: Re: st: test difference in quintiles
-cendif- by Roger Newson, is part of the -somersd- package,
downloadable from SSC (type "ssc install somersd". It computes CI's
for, and tests, differences between medians. You will need large
sample sizes to test the difference between 5th percentiles with
reasonable power. What do you mean by "number of observations in the
5th percentile"? You should not group data prior to the analysis.
-Steve
On Wed, Jun 10, 2009 at 10:57 AM, Susanne
Neckermann<[email protected]> wrote:
> Hi!
>
> I want to test whether the difference in the fifth percentile of two
> distributions is significant.
> In principle, this test should be similar to the "median" (thats the name of
> the command) test already implemented in Stata, but unfortunately I cannot
> program .ado files and I could not find a user-written procedure.
> Any idea how to go about this?
> P.S. the number of observations in the fifth percentile is pretty low
> (around 6), so one might have to use something with bootstrap or
> permutations.
>
> Thank you so much in advance
> susanne
>
>
>
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>
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