[R] Problems with basic loop

[Peter Dalgaard]

Michael Pearmain wrote:
> Thanks for the reply Peter,
>
>   
>> 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
>>     
Ah. Notwithstanding other remarks, that one would be due to

yT<-x[,"True"]/colSums(x)

The divisor here is a vector of length two!! So every 2nd time you are dividing with the wrong thing and in one case that gets you a relative frequency bigger than 1...


>> 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
>>
>>     
I still don't see where the notion of power needs to come in. Shouldn't
the effect size for  "The questions that I am looking at later" enter
somehow? If you just want to know whether there is age-dependent
exposure, how about

chisq.test(SURVEY$n_0,exposed)

(or wilcox.test if you want something that takes the group ordering into
account, or prop.trend.test). If, for better or worse, you want to test
single rows, you could use

chisq.test(SURVEY$n_0 == "35-44",exposed)

etc.

>>
>> 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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-- 
   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