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Re: st: nptrend question


From   Ricardo Ovaldia <ovaldia@yahoo.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: nptrend question
Date   Fri, 21 Nov 2003 05:35:42 -0800 (PST)

Thank you Roger. It took me a while to understand your
article, but I think I got it.

Best,

Ricardo.

--- Roger Newson <roger.newson@kcl.ac.uk> 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: roger.newson@kcl.ac.uk
> Website: http://www.kcl-phs.org.uk/rogernewson
> 
> Opinions expressed are those of the author, not the
> institution.
> 
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=====
Ricardo Ovaldia, MS
Statistician 
Oklahoma City, OK

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