From: Michael Pearmain <mpearmain_at_google.com>

Date: Fri, 20 Jun 2008 11:53:19 +0100

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 Fri 20 Jun 2008 - 10:57:15 GMT

Date: Fri, 20 Jun 2008 11:53:19 +0100

*> I did just see that i had put the first error message,(agreed rather an
**> obvious error) in and not the second i received
**>
**> Warning message:
**> In asin(sqrt(yF[i])) : NaNs produced
**>
**> The reason i'm looking at this is advert exposure True and False.
**>
**> I'm inspecting age to asses weather or not to weight data in order to
**> normalise groups for later questions,
**> The questions that i am looking at later on are not scale based questions
**> so i cannot perform t-tests on these, so i thought the only viable way was
**> to look at z-tests for proportions to check for post-hoc differences
**>
**> Any advise on other methods would be gratefully taken
**>
**>
**>
**> On Fri, Jun 20, 2008 at 11:14 AM, Peter Dalgaard <P.Dalgaard_at_biostat.ku.dk>
**> wrote:
**>
*

>> Michael Pearmain wrote:

*>> > I'm having trouble creating a looping variable and i can't see wher ethe
**>> > problem arises from any hep gratfully appreciated
**>> >
**>> > First create a table
**>> >
**>> > x<-table(SURVEY$n_0,exposed)
**>> >
**>> >> x
**>> >>
**>> > exposed
**>> > False True
**>> > Under 16 24 1
**>> > 16-19 68 9
**>> > 20-24 190 37
**>> > 25-34 555 204
**>> > 35-44 330 87
**>> > 45-54 198 65
**>> > 55-64 67 35
**>> > 65+ 10 8
**>> >
**>> > Now ectors to store counts and column proportions
**>> >
**>> >
**>> >> xT<-x[,"True"]
**>> >> xF<-x[,"False"]
**>> >> yT<-x[,"True"]/colSums(x)
**>> >> yF<-x[,"False"]/colSums(x)
**>> >>
**>> >
**>> > check length for dynamic looping
**>> >
**>> >> length(yT)
**>> >>
**>> > [1] 8
**>> >
**>> > now create loop
**>> >
**>> >> for(i in 1:length(yT)){
**>> >>
**>> > +
**>> pwr.2p2n.test(2*(asin(sqrt(yT[i]))-asin(sqrt(yF[i]))),n1=xT[i],n2=xF[i])
**>> > + }
**>> > Error in pwr.2p2n.test(2 * (asin(sqrt(yT[i])) - asin(sqrt(yF[i]))), n1 =
**>> > xT[i], :
**>> > number of observations in the first group must be at least 2
**>> >
**>> > this confuses me as if i enter the data as values the procedure works?
**>> >
**>> > Thanks in advance
**>> >
**>> Er, the first row "under 16" has a count of 1 in the "True" column and
**>> it confuses you that you get an error saying that you need at least 2??
**>>
**>> But what looks _really_ confused is what you are trying to do in the
**>> first place: The p's you are passing to pwr.2p2n are the empirical
**>> relative frequencies of the individual age groups. This sort of reverses
**>> cause and effect (presumably the exposure does not cause middle age) and
**>> it is pretty odd to compare a particular row in a table with everything
**>> else jumbled together but worse, it is post-hoc power calculation, which
**>> is just a plain Bad Idea (as several people have pointed out before).
**>>
**>> --
**>> O__ ---- Peter Dalgaard ุster 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
**>>
**>>
**>>
**>
**>
**> --
**> Michael Pearmain
**> Senior Statistical Analyst
**>
**>
**> 1st Floor, 180 Great Portland St. London W1W 5QZ
**> t +44 (0) 2032191684
**> mpearmain_at_google.com
**> mpearmain_at_doubleclick.com
**>
**>
**> Doubleclick is a part of the Google group of companies
*

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