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st: RE: RE: -powercal-


From   "Mak, Timothy" <Timothy.Mak@iop.kcl.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: RE: -powercal-
Date   Fri, 9 Feb 2007 17:41:29 -0000

Hi Roger, 

My hypothesis is that the proportion of attendance for the control group
is 0.5, and the proportion for the experimental group is 0.8. How do you
translate that into a z-test? 

Thanks, 

Tim  

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Newson, Roger
B
Sent: 09 February 2007 14:44
To: statalist@hsphsun2.harvard.edu
Subject: st: RE: -powercal-

I think you probably could use -powercal- to do these power
calculations. The formulas are given in Newson (2004). However, to use
these formulas, you must first specify the kind of effect you intend to
measure (eg a ratio between proportions or a difference between
proportions or a difference between odds), and the groups you intend to
compare. These specifications not clear from your email (quoted below).

I hope this helps.

Best wishes

Roger


References

Newson R. Generalized power calculations for generalized linear models
and more. The Stata Journal 2004; 4(4): 379-401. Download
pre-publication draft from http://www.imperial.ac.uk/nhli/r.newson/

Roger Newson
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
www.imperial.ac.uk/nhli/r.newson/

Opinions expressed are those of the author, not of the institution.

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Mak, Timothy
Sent: 09 February 2007 14:06
To: statalist@hsphsun2.harvard.edu
Subject: st: -powercal-

Hi all, 

I would like to use calculate sample size for an RCT whose outcome will
be assessed by Binomial regression. Basically it's the proportion of
treatment sessions that are attended. Some will be assigned 3 sessions,
some 6, and some 12. Say the proportions are 60%, 20%, and 20%. Assuming
equal proportion between treatment and control group, I wonder if I
could use -powercal- to do the calculations instead of running a
simulation. 

Help would be much appreciated. 

Tim

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