Thank you Roger. It took me a while to understand your
article, but I think I got it.
Best,
Ricardo.
--- Roger Newson <[email protected]> wrote:
> At 10:04 18/11/03 -0800, Ricardo Ovaldia wrote:
> >Dear Stata users,
> >
> >I used -nptrend- to perform a nonparametric test
> for
> >trend across ordered groups. I assumed that because
> it
> >was non-parametric the way that the groups were
> coded
> >was not important. I coded a severity of illness
> >variable as follows:
> >
> >0 not severe
> >1 mildly severe
> >2 severe
> >3 most severe
> >
> >I run the -nptrend- an obtained:
> >
> >. nptr died, by(severe)
> >
> > severe score obs sum of ranks
> > 0 0 129 19828
> > 1 1 88 13737
> > 2 2 66 11148
> > 3 3 39 7290
> >
> > z = 2.75, chi-squared(1) = 7.57
> > P>|z| = 0.0059
> >
> >
> >When I recoded the severity variable as:
> >
> > 0 not severe
> >.5 mildly severe
> >.7 severe
> > 1 most severe
> >
> >but maintaining the order, to my surprise I
> obtained:
> >
> >. nptr died, by(severe)
> >
> > severe score obs sum of ranks
> > 0 0 129 19828
> > .5 .5 88 13737
> > .7 .7 66 11148
> > 1 1 39 7290
> >
> > z = 2.52, chi-squared(1) = 6.35
> > P>|z| = 0.0117
> >
> >
> >I though that this nonparametric test would be
> >invariant to the scale and only affected by the
> >relative ordering of the categories.
> >
> >Is there a test for trend that is not affected by
> the
> >magnitude of the class variable values?
>
> Yes. My own -somersd- package, downloadable from
> SSC, calculates confidence
> intervals for Somers' D and Kendall's tau-a, both of
> which are ordinal
> trend measures unaffected by the magnitude of the
> class variable values. A
> general review of these parameters behind
> "nonparametric" statistics is my
> Stata Journal article (Newson, 2002). A
> pre-publication draft of this can
> be downloaded from my website at
> http://www.kcl-phs.org.uk/rogernewson/
> either using a browser or using the Stata -net-
> command. The -somersd-
> package is distributed with .pdf manuals, which are
> complementary to my SJ
> article.
>
> I hope this helps.
>
> Roger
>
> References
>
> Newson R. Parameters behind "nonparametric"
> statistics: Kendall's tau,
> Somers' D and median differences. The Stata Journal
> 2002; 2(1): 45-64.
>
>
> --
> Roger Newson
> Lecturer in Medical Statistics
> Department of Public Health Sciences
> King's College London
> 5th Floor, Capital House
> 42 Weston Street
> London SE1 3QD
> United Kingdom
>
> Tel: 020 7848 6648 International +44 20 7848 6648
> Fax: 020 7848 6620 International +44 20 7848 6620
> or 020 7848 6605 International +44 20 7848 6605
> Email: [email protected]
> Website: http://www.kcl-phs.org.uk/rogernewson
>
> Opinions expressed are those of the author, not the
> institution.
>
> *
> * For searches and help try:
> *
> http://www.stata.com/support/faqs/res/findit.html
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> * http://www.ats.ucla.edu/stat/stata/
=====
Ricardo Ovaldia, MS
Statistician
Oklahoma City, OK
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