From: Peter Dalgaard <P.Dalgaard_at_biostat.ku.dk>

Date: Tue, 17 Jun 2008 18:56:51 +0200

Date: Tue, 17 Jun 2008 18:56:51 +0200

Michael Pearmain wrote:

> Yes my mistake,

*>
**> I looked at the pwr.2p2n.test but i cannot place both n's and both p
**> values to determine the sig value
**> e,g *pwr.2p2n.test(h = , n1 = , n2 = , sig.level = , power = )
**>
**> or am i missing someting obvious?
**>
**> *
*

Not quite obvious, but h is the effect size and is a function of p1 &
p2, after arcsine transformation. The documentation for the pwr package
is a bit short on information on _which_ arcsine transformation that is,
though. As far as I can see, it is 2*asin(sqrt(p)), since

*> pwr.2p2n.test(2*(asin(sqrt(.5))-asin(sqrt(.7))),n1=200,n2=200)
*

difference of proportion power calculation for binomial distribution (arcsine transformation)

h = 0.4115168 n1 = 200 n2 = 200 sig.level = 0.05 power = 0.984427 alternative = two.sided

NOTE: different sample sizes

*> power.prop.test(p1=.5,p2=.7,n=200)
*

Two-sample comparison of proportions power calculation

n = 200 p1 = 0.5 p2 = 0.7 sig.level = 0.05 power = 0.9849271 alternative = two.sided

NOTE: n is number in *each* group

> *i did the sam ein SPSS using a macro and the following code:

*>
**> COMPUTE n1 = Control_MAX .
**> COMPUTE n2 = Exposed_max.
**> COMPUTE x1 = Control.
**> COMPUTE x2 = Exposed.
**>
**> COMPUTE p1 = x1/n1.
**> COMPUTE p2 = x2/n2.
**> COMPUTE phat = (x1 + x2) / (n1 + n2).
**> COMPUTE SE_phat = SQRT(phat * (1 - phat) * ((1/n1) + (1/n2))).
**> COMPUTE z = (p1 - p2) /SE_phat.
**> COMPUTE SIGz_2TL = 2 * (1 - CDFNORM(ABS(z))).
**> COMPUTE SIGz_LTL = CDFNORM(Z).
**> COMPUTE SIGz_UTL = 1 - CDFNORM(Z).
**> COMPUTE SIG_Level = ABS(1-(1-CDFNORM(z))*2).
**> Compute p1p = p1*100.
**> Compute p2p = p2*100.
**> compute diff = p2p-p1p.
**> EXE.
**> Var lab p1p "Control Group %".
**> Var lab p2p "Exposed Group %".
**>
**>
**>
**> *
**>
**>
**> On Tue, Jun 17, 2008 at 5:13 PM, Peter Dalgaard
**> <P.Dalgaard_at_biostat.ku.dk <mailto:P.Dalgaard_at_biostat.ku.dk>> wrote:
**>
**> Michael Pearmain wrote:
**> > Hi All,
**> >
**> > I have a table based on ordial data and i want to compare
**> proportions and
**> > i've seen in the pwr package i can use
**> > power.prop.test
**> >
**> > however i want to find out what the sig. value is based on
**> n1,n2,p1,p2 and
**> > this package doesn't contain this..
**> > Does anyone know of a package that does or is it a case of
**> writting a
**> > function specifically for this?
**> >
**> >
**> I think your wired got crossed somewhere:
**>
**> power.prop.test is not from the pwr package; however, pwr does
**> contain pwr.2p2n.test, which looks like it does exactly what you want!
**>
**>
**> > Many thanks in advance
**> >
**> > [[alternative HTML version deleted]]
**> >
**> > ______________________________________________
**> > R-help_at_r-project.org <mailto:R-help_at_r-project.org> mailing list
**> > https://stat.ethz.ch/mailman/listinfo/r-help
**> > PLEASE do read the posting guide
**> http://www.R-project.org/posting-guide.html
**> > and provide commented, minimal, self-contained, reproducible code.
**> >
**>
**>
**> --
**> O__ ---- Peter Dalgaard Øster Farimagsgade 5, Entr.B
**> c/ /'_ --- Dept. of Biostatistics PO Box 2099, 1014 Cph. K
**> (*) \(*) -- University of Copenhagen Denmark Ph: (+45)
**> 35327918
**> ~~~~~~~~~~ - (p.dalgaard_at_biostat.ku.dk
**> <mailto:p.dalgaard_at_biostat.ku.dk>) FAX: (+45) 35327907
**>
**>
**>
**>
**>
**> --
**> Michael Pearmain
**> Senior Statistical Analyst
**>
**>
**> 1st Floor, 180 Great Portland St. London W1W 5QZ
**> t +44 (0) 2032191684
**> mpearmain_at_google.com <mailto:mpearmain_at_google.com>
**> mpearmain_at_doubleclick.com <mailto:mpearmain_at_doubleclick.com>
**>
**>
**> Doubleclick is a part of the Google group of companies
*

-- O__ ---- Peter Dalgaard Øster Farimagsgade 5, Entr.B c/ /'_ --- Dept. of Biostatistics PO Box 2099, 1014 Cph. K (*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918 ~~~~~~~~~~ - (p.dalgaard_at_biostat.ku.dk) FAX: (+45) 35327907 ______________________________________________ R-help_at_r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.Received on Tue 17 Jun 2008 - 17:17:23 GMT

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