Re: st: cendif

["Roger B. Newson" <r.newson@imperial.ac.uk>]

If you want to compare 2 intervention groups separately with the control 
using -somersd- and -cendif-, then you need to use an -if- qualifier for 
each intervention-control pair. If your grouping variable is -grupo-, 
with values 0 for control, 1 for Intervention 1, and 2 for Intervention 
2, then you can compare interventions 1 and 2 with the control by typing:

xi i.grupo, noomit
somersd grupo T6convulsion if inlist(grupo,0,1), transf(z) tdist 
cluster(medcod3);
cendif T6convulsion if inlist(grupo,0,1), by(_Igrupo_0) transf(z) tdist 
cluster(medcod3);
somersd grupo T6convulsion if inlist(grupo,0,2), transf(z) tdist 
cluster(medcod3);
cendif T6convulsion if inlist(grupo,0,2), by(_Igrupo_0) transf(z) tdist 
cluster(medcod3);

This will give the 2 median intervention-control differences in outcome, 
with confidence limits, and also confidence linits, and P-values, for 
the Somers' D parameters of outcome with respect to each intervention 
compared to the control group.

I hope this helps. Let me know if you have any more queries.

Best wishes

Roger


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: r.newson@imperial.ac.uk
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.

On 22/11/2012 13:55, alfonsa leiva wrote:
> Dear stata user
>
> In the between group analisis (control vs Intervention1/ control vs Intervention2/Intervention1 vs Intervention2)of a three arm cluster trial,
>
>   I can get p- values by somersd:
>
> xi, noomit: somersd   T6convulsion i.grupo, taua transf(z) tdist cluster(medcod3)
>
> testparm _I*
>
>
> . testparm _I*
>
>   ( 1)  _Igrupo_1 = 0
>   ( 2)  _Igrupo_2 = 0
>         Constraint 2 dropped
>
>         F(  1,    46) =    7.02
>              Prob > F =    0.0110
>
>
>
> Could i use also Hodges-lehman estimates by cendiff to report differences between groups?
>
>
> xi,noomit: cendif   T6irritabilidad , transf(z) tdist cluster(medcod3) by(grupo)
>
>
>   Percent    Pctl_Dif     Minimum     Maximum
>          50           0           0           0
>
>
> I apreciatte any comment
>
>
>
> Thanks
> *
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