[R] ci.pd() (Epi) and Newcombe method

[(Ted Harding)]

On 06-Feb-08 17:09:59, Peter Dalgaard wrote:
> (Ted Harding) wrote:
>> Greetings!
>>
>> I suspect that there is an error in the code for the
>> function ci.pd() in the Epi package.
>>   
> Any particular reason not to include the maintainer among the 
> recipients??

Oversight, and haste! Thank you for copying your reply
(with my original message) to him. And apologies to Bendix.

Ted.

> Not sure whether Bendix is subscribed.
> 
>     -p
> 
>> This function is for computing confidence intervals
>> for a difference of proportions between two independent
>> groups of 0/1 responses, and implements the Newcombe
>> ("Nc") method and the Agrasti-Caffo "AC" method.
>>
>> I think there is an error in the computation for the
>> Newcombe method.
>>
>> Specifically, where the code says
>>
>> # Put the data in the right form
>>   x1 <- aa; n1 <- aa+cc; p1 <- x1/n1
>>   x2 <- bb; n2 <- bb+dd; p2 <- x2/n2
>>   pd <- x1/n1 - x2/n2
>>   z <- qnorm(1-alpha/2)
>>   zz <- qchisq(1-alpha/2,1)
>>
>> I believe zz (which is used in the Newcombe method,
>> while z is used for Agresti-Caffo) should be
>>
>>   zz <- qchisq(1-alpha,1)
>>
>> (though I think they could just as well have written
>> xx <- z^2), since z^2 seems to be what they want for the
>> Newcombe method, and qnorm(1-alpha/2)^2 = qchisq(1-alpha,1)
>>
>> This was noticed as a result of comparing results from
>> ci.pd() with results from the RDCI module in STATA (run
>> by someone else, since I don't have access to STATA).
>> With the original code in ci.pd(), the Newcombe results
>> were different between ci.pd() and STATA. After making
>> the above change, they agreed.
>>
>> I would be most grateful if anyone who has also been
>> down this path could confirm whether I am correct.
>> Since I don't have access to Newcombe's original
>> paper at the moment either, I am not able to check
>> the code against his own description of the method!
>>
>> With thanks,
>> Ted.
>>
>> --------------------------------------------------------------------
>> E-Mail: (Ted Harding) <Ted.Harding at manchester.ac.uk>
>> Fax-to-email: +44 (0)870 094 0861
>> Date: 06-Feb-08                                       Time: 16:58:19
>> ------------------------------ XFMail ------------------------------
>>
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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
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
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--------------------------------------------------------------------
E-Mail: (Ted Harding) <Ted.Harding at manchester.ac.uk>
Fax-to-email: +44 (0)870 094 0861
Date: 06-Feb-08                                       Time: 17:29:34
------------------------------ XFMail ------------------------------