[R] Fisher's r to z' transformation - help needed
Peter Dalgaard
P.Dalgaard at biostat.ku.dk
Wed May 23 15:00:12 CEST 2007
Mike White wrote:
> I am trying to use Fisher's z' transformation of the Pearson's r but the
> standard error does not appear to be correct. I have simulated an example
> using the R code below. The z' data appears to have a reasonably normal
> distribution but the standard error given by the formula 1/sqrt(N-3) (from
> http://davidmlane.com/hyperstat/A98696.html) gives a different results than
> sd(z). Can anyone tell me where I am going wrong?
>
>
You're not calculating 3000 independent sample correlations. You are
calculating the correlation of each row of "dat" with the column means
in one case, and a bunch of pairwise corrrelations in the other. In
addition, and this may be the important part, you are generating
"correlation" not by using a sample from a two-dimensional normal but by
adding in specific means.
Conrast
> r <- replicate(30000, {Z<-rnorm(10,55,30);cor(rnorm(10)+Z,rnorm(10)+Z)})
> z <- 0.5*log((1+r)/(1-r))
> sd(z)
[1] 0.3648677
> 1/sqrt(7)
[1] 0.3779645
with
> r <- replicate(30000, cor(rnorm(10)+10*(1:10), rnorm(10)+10*(1:10)))
> z <- 0.5*log((1+r)/(1-r))
> sd(z)
[1] 0.2662451
> library(amap)
>
> ## SIMULATED DATA #########################################################
> p<-10
> n<-3000
> means<-1000*c(1:p)
> SDs<-rep(100,p)
> set.seed(1)
> dat<-mapply(rnorm, mean=means, sd=SDs, n=n)
> colnames(dat)<-paste("V",1:p, sep="")
> rownames(dat)<-1:n
> # calculate centroid of simulated data
> dat.mean<-apply(dat,2,mean)
> # calculated Pearson's r to centroid
> r<-apply(dat,1,cor, y=dat.mean)
> plot(density(r))
> # Fisher's z' transformation
> z<-0.5*log((1+r)/(1-r))
> plot(density(z))
> sd(z)
> # [1] 0.2661779
>
> 1/sqrt(p-3)
> # [1] 0.3779645
>
> ## alternatively use comparisons for all possible pairs
> ## Centred Pearson's r on raw data
> r<-1-Dist(dat,"corr")
> plot(density(r))
> z<-0.5*log((1+r)/(1-r))
> plot(density(z))
> sd(z)
> # [1] 0.2669787
> 1/sqrt(p-3)
> # [1] 0.3779645
>
> Many thanks
> Mike White
>
> ______________________________________________
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> and provide commented, minimal, self-contained, reproducible code.
>
--
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
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