[R] Pearson corelation and p-value for matrix

[Marc Schwartz]

John,

Interesting test. Thanks for pointing that out.

You are right, there is a knee-jerk reaction to avoid loops, especially
nested loops.

On the indexing of rows, I did that because Dren had indicated in his
initial post:

 "I was trying to evaluate the pearson correlation and the p-values 
  for an nxm matrix, where each row represents a vector.
  One way to do it would be to iterate through each row, and find its
  correlation value( and the p-value) with respect to the other rows."

So I ran the correlations by row, rather than by column.

Thanks again. Good lesson.

Marc

On Fri, 2005-04-15 at 21:36 -0400, John Fox wrote:
> Dear Mark,
> 
> I think that the reflex of trying to avoid loops in R is often mistaken, and
> so I decided to try to time the two approaches (on a 3GHz Windows XP
> system).
> 
> I discovered, first, that there is a bug in your function -- you appear to
> have indexed rows instead of columns; fixing that:
> 
> cor.pvals <- function(mat)
> {
>   cols <- expand.grid(1:ncol(mat), 1:ncol(mat))
>   matrix(apply(cols, 1,
>                function(x) cor.test(mat[, x[1]], mat[, x[2]])$p.value),
>          ncol = ncol(mat))
> }
> 
> 
> My function is cor.pvalues and yours cor.pvals. This is for a data matrix
> with 1000 observations on 100 variables:
> 
> > R <- diag(100)
> > R[upper.tri(R)] <- R[lower.tri(R)] <- .5
> > library(mvtnorm)
> > X <- rmvnorm(1000, sigma=R)
> > dim(X)
> [1] 1000  100
> > 
> > system.time(cor.pvalues(X))
> [1] 5.53 0.00 5.53   NA   NA
> > 
> > system.time(cor.pvals(X))
> [1] 12.66  0.00 12.66    NA    NA
> > 
> 
> I frankly didn't expect the advantage of my approach to be this large, but
> there it is.
> 
> Regards,
>  John
> 
> --------------------------------
> John Fox
> Department of Sociology
> McMaster University
> Hamilton, Ontario
> Canada L8S 4M4
> 905-525-9140x23604
> http://socserv.mcmaster.ca/jfox 
> -------------------------------- 
> 
> > -----Original Message-----
> > From: Marc Schwartz [mailto:MSchwartz at MedAnalytics.com] 
> > Sent: Friday, April 15, 2005 7:08 PM
> > To: John Fox
> > Cc: 'Dren Scott'; R-Help
> > Subject: RE: [R] Pearson corelation and p-value for matrix
> > 
> > Here is what might be a slightly more efficient way to get to John's
> > question:
> > 
> > cor.pvals <- function(mat)
> > {
> >   rows <- expand.grid(1:nrow(mat), 1:nrow(mat))
> >   matrix(apply(rows, 1,
> >                function(x) cor.test(mat[x[1], ], mat[x[2], 
> > ])$p.value),
> >          ncol = nrow(mat))
> > }
> > 
> > HTH,
> > 
> > Marc Schwartz
> > 
> > On Fri, 2005-04-15 at 18:26 -0400, John Fox wrote:
> > > Dear Dren,
> > > 
> > > How about the following?
> > > 
> > >  cor.pvalues <- function(X){
> > >     nc <- ncol(X)
> > >     res <- matrix(0, nc, nc)
> > >     for (i in 2:nc){
> > >         for (j in 1:(i - 1)){
> > >             res[i, j] <- res[j, i] <- cor.test(X[,i], X[,j])$p.value
> > >             }
> > >         }
> > >     res
> > >     }
> > > 
> > > What one then does with all of those non-independent test 
> > is another 
> > > question, I guess.
> > > 
> > > I hope this helps,
> > >  John
> > 
> > > > -----Original Message-----
> > > > From: r-help-bounces at stat.math.ethz.ch 
> > > > [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Dren Scott
> > > > Sent: Friday, April 15, 2005 4:33 PM
> > > > To: r-help at stat.math.ethz.ch
> > > > Subject: [R] Pearson corelation and p-value for matrix
> > > > 
> > > > Hi,
> > > >  
> > > > I was trying to evaluate the pearson correlation and the p-values 
> > > > for an nxm matrix, where each row represents a vector. 
> > One way to do 
> > > > it would be to iterate through each row, and find its correlation 
> > > > value( and the p-value) with respect to the other rows. Is there 
> > > > some function by which I can use the matrix as input? 
> > Ideally, the 
> > > > output would be an nxn matrix, containing the p-values 
> > between the 
> > > > respective vectors.
> > > >  
> > > > I have tried cor.test for the iterations, but couldn't find a 
> > > > function that would take the matrix as input.
> > > >  
> > > > Thanks for the help.
> > > >  
> > > > Dren
> > 
> >
> 
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