Hi Kingsford,
There is more structure to a correlation matrix than that meets the eye! Your
method will produce a matrix R that looks "like" a correlation matrix, but
beware - it is an impostor!
You can obtain a valid correlation matrix, Q, from the impostor R by using the
`nearPD' function in the "Matrix" package, which finds the positive definite
matrix Q that is "nearest" to R. However, note that when R is far from a
positive-definite matrix, this step may give a Q that does not have the desired
property.
require(Matrix)
R <- matrix(runif(16), ncol=4)
R <- (R * lower.tri(R)) + t(R * lower.tri(R))
diag(R) <- 1
eigen(R)$val
Q <- nearPD(R, posd.tol=1.e-04)$mat
eigen(Q)$val
max(abs(Q - R)) # maximum discrepancy between R and Q
Another easy way to produce a valid correlation matrix is:
R <- matrix(runif(36), ncol=6)
RtR <- R %*% t(R)
Q <- cov2cor(RtR)
But this does not have the property that the correlations are uniformly
distributed.
Hope this helps,
Ravi.
____________________________________________________________________
Ravi Varadhan, Ph.D.
Assistant Professor,
Division of Geriatric Medicine and Gerontology
School of Medicine
Johns Hopkins University
Ph. (410) 502-2619
email: rvarad...@jhmi.edu
----- Original Message -----
From: Kingsford Jones <kingsfordjo...@gmail.com>
Date: Friday, August 28, 2009 10:12 pm
Subject: Re: [R] how to generate a random correlation matrix with
restrictions
To: Ning Ma <pnin...@gmail.com>
Cc: r-help@r-project.org
> Ahh -- Mark Leeds just pointed out off-list that it was supposed to be
> a correlation matrix.
>
> Perhaps
>
> R <- matrix(runif(10000), ncol=100)
> R <- (R * lower.tri(R)) + t(R * lower.tri(R))
> diag(R) <- 1
>
> These are of course uniformly distributed positive correlations.
>
> Kingsford
>
>
> On Fri, Aug 28, 2009 at 7:36 PM, Kingsford
> Jones<kingsfordjo...@gmail.com> wrote:
> > R <- matrix(runif(10000), ncol=100)
> >
> > hth,
> >
> > Kingsford Jones
> >
> > On Fri, Aug 28, 2009 at 7:26 PM, Ning Ma<pnin...@gmail.com> wrote:
> >> Hi,
> >>
> >> How can I generate a random 100x100 correlation matrix, R={r_ij},
> >> where about 10% of r_ij are greater than 0.9
> >>
> >> Thanks in advance.
> >>
> >> ______________________________________________
> >> R-help@r-project.org mailing list
> >>
> >> PLEASE do read the posting guide
> >> and provide commented, minimal, self-contained, reproducible code.
> >>
> >
>
> ______________________________________________
> R-help@r-project.org mailing list
>
> PLEASE do read the posting guide
> and provide commented, minimal, self-contained, reproducible code.
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