[R] solving a structural equation model using sem or other package

Daniel Nordlund res90sx5 at verizon.net
Wed Jan 24 16:47:59 CET 2007


Thanks for the help.  I missed the significance of the section you quoted below from the help.  That does indeed solve the problem.


Dan Nordlund
Bothell, WA  USA

> -----Original Message-----
> From: David Barron [mailto:mothsailor at googlemail.com]
> Sent: Wednesday, January 24, 2007 3:32 AM
> To: Daniel Nordlund; r-help
> Subject: Re: [R] solving a structural equation model using sem or other package
> This is an extract from the sem help page, which deals with your situation:
> S covariance matrix among observed variables; may be input as a
> symmetric matrix, or as a lower- or upper-triangular matrix. S may
> also be a raw (i.e., ``uncorrected'') moment matrix — that is, a
> sum-of-squares-and-products matrix divided by N. This form of input is
> useful for fitting models with intercepts, in which case the moment
> matrix should include the mean square and cross-products for a unit
> variable all of whose entries are 1; of course, the raw mean square
> for the unit variable is 1. Raw-moment matrices may be computed by
> raw.moments.
> On 24/01/07, Daniel Nordlund <res90sx5 at verizon.net> wrote:
> > I am trying to work my way through the book "Singer, JD and Willett, JB, Applied
> Longitudinal Data Analysis. Oxford University Press, 2003"  using R.  I have the SAS
> code and S-Plus code from the UCLA site (doesn't include chapter 8 or later
> problems).  In chapter 8, there is a structural equation/path model which can be
> specified for the sem package as follows
> >
> > An equivalent specification in SAS produces the solution presented in the book.  The
> variable cons is a constant vector of 1's.  The problem with the sem package is that the
> covariance matrix which includes the variable cons is singular and sem says so and
> will not continue.  Is there an alternative way to specify this problem for sem to obtain
> a solution?  If not, is there another package that would produce a solution?
> >
> > Thanks,
> >
> > Dan Nordlund
> > Bothell, WA
> >
> > ______________________________________________
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> > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> > and provide commented, minimal, self-contained, reproducible code.
> >
> --
> =================================
> David Barron
> Said Business School
> University of Oxford
> Park End Street
> Oxford OX1 1HP

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