Re: [R] Problems with basic loop

From: Peter Dalgaard <P.Dalgaard_at_biostat.ku.dk>
Date: Fri, 20 Jun 2008 12:14:56 +0200

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             Ă˜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

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