[R-sig-Epi] Transform function

[BXC (Bendix Carstensen)]

Ralf,
there is a typo in the notes; it should read:
 
births<- transform(births,hyp=factor(hyp,labels=c("normal","hyper")))

"labels" is the argument to factor() specifying the text labels associated with the levels.
Since the levels per default are the ordered values, 0 of the original corrsponds to "normal"
and 1 to "hyper", try:

data(births)
with( births, table(hyp,factor(hyp,labels=c("normal","hyper")) )
 
Best,
Bendix

> -----Original Message-----
> From: r-sig-epi-bounces at stat.math.ethz.ch 
> [mailto:r-sig-epi-bounces at stat.math.ethz.ch] On Behalf Of Ralf Finne
> Sent: Sunday, June 10, 2007 10:06 PM
> To: r-sig-epi at stat.math.ethz.ch
> Subject: [R-sig-Epi] Transform function
> 
> Hi all epidemiologist.
> 
> I visited the exellent course SPE in Tartu in May.
> One small detail created a question.  From 
> www.pubhealth.ku.dk/~bxc/SPE/ I found "A short introduction 
> to R" by Michael Hills, Martyn Plummer and Bendix Carstensen.
> On page 6 there is an example of the function "transform":
> 
> births<- transform(births,hyp=factor(hyp,labels(c("normal","hyper"))))
> 
> Running the program:
> 
> > library(Epi)
> > data(births)
> > str(births)
> 'data.frame':   500 obs. of  8 variables:
>  $ id     : num  1 2 3 4 5 6 7 8 9 10 ...
>  $ bweight: num  2974 3270 2620 3751 3200 ...
>  $ lowbw  : num  0 0 0 0 0 0 0 0 0 0 ...
>  $ gestwks: num  38.5   NA 38.2 39.8 38.9 ...
>  $ preterm: num  0 NA 0 0 0 0 0 0 0 0 ...
>  $ matage : num  34 30 35 31 33 33 29 37 36 39 ...
>  $ hyp    : num  0 0 0 0 1 0 0 0 0 0 ...
>  $ sex    : num  2 1 2 1 1 2 2 1 2 1 ...
> 
> > births<- 
> transform(births,hyp=factor(hyp,labels(c("normal","hyper")))
> )
> > str(births)
> gives:
> 
> 'data.frame':   500 obs. of  8 variables:
>  $ id     : num  1 2 3 4 5 6 7 8 9 10 ...
>  $ bweight: num  2974 3270 2620 3751 3200 ...
>  $ lowbw  : num  0 0 0 0 0 0 0 0 0 0 ...
>  $ gestwks: num  38.5   NA 38.2 39.8 38.9 ...
>  $ preterm: num  0 NA 0 0 0 0 0 0 0 0 ...
>  $ matage : num  34 30 35 31 33 33 29 37 36 39 ...
>  $ hyp    : Factor w/ 2 levels "1","2": NA NA NA NA 1 NA NA 
> NA NA NA ...
>  $ sex    : num  2 1 2 1 1 2 2 1 2 1 ...
> 
> The item births$hyp is transformed from numeric into a factor 
> in a wrong way.
> 
> The worst thing is that in the beginning in Tartu the 
> transformation was sometimes correct and sometimes wrong.  Am 
> I doing a mistake or is there a memory collision in the R 
> system. I have tested on two PC:s with XP on R versions 2.4.1 
> and 2.5.0 and Epi package on two versions the newest and an older one.
> 
> By the way how do I find the data files included in the Epi-package? 
> The functions I
> find by.
> search()
> ls(2)
> 
> Thank you and good luck with the great work you are doing.
> 
> Ralf Finne.
> Svenska Yrkeshögskolan
> Vasa Finland.
> +358 505 539643
> 
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