R help - Aug 2013 - Structural equation models (SEM) for count data / poisson distribution

Dear R community,

I am constructing structural equation models in R and I have tried both 
the sem and lavaan packages. I have count data (numbers of plants in 
this case) that I would like to use as an endogenous variable. The 
poisson distribution seems appropriate for these data, but I can't seem 
to figure out how best to handle this within either package, lavaan or 
sem, though I have noticed that both packages are adding lots of 
options/capability to their core sem functions, so perhaps I have missed 
something.

Could someone recommend a reasonable approach for handling count data 
with SEM in R, in either lavaan or sem?

Thanks in advance for comments and suggestions,
Stella

-- 
Stella Copeland
PhD Candidate
Environmental Science & Policy
UC Davis

Dear Stella,

The sem package doesn't make provision for count data. Because the sem()
function allows the user to specify an arbitrary objective function, if you know
the likelihood for the model that you want to fit, you could in principle write
a corresponding objective function, but this would be a nontrivial task.

Another approach might be to use the multinormal likelihood along with robust
standard errors after transforming the counts by, say, a square-root
transformation, but this isn't promising if there are many 0 counts.

I hope this helps,
 John

On Tue, 06 Aug 2013 09:51:56 -0700
 Stella Copeland <scopeland at ucdavis.edu> wrote:
> Dear R community,
> 
> I am constructing structural equation models in R and I have tried both the
sem and lavaan packages. I have count data (numbers of plants in this case) that
I would like to use as an endogenous variable. The poisson distribution seems
appropriate for these data, but I can't seem to figure out how best to
handle this within either package, lavaan or sem, though I have noticed that
both packages are adding lots of options/capability to their core sem functions,
so perhaps I have missed something.
> 
> Could someone recommend a reasonable approach for handling count data with
SEM in R, in either lavaan or sem?
> 
> Thanks in advance for comments and suggestions,
> Stella
> 
> -- 
> Stella Copeland
> PhD Candidate
> Environmental Science & Policy
> UC Davis
> 
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