[R] esthetics --- extending the lm command to fixed effects?
Achim.Zeileis at uibk.ac.at
Thu May 20 18:03:06 CEST 2010
On Thu, 20 May 2010, ivo welch wrote:
> hi thomas---
> thanks for the answer. the problem with the "+factor(fmid)" is not just
> that it provides uninteresting coefficients and that it eats more memory,
> but that it is also MUCH slower when there are (hundred of) thousands of
> fixed effects.
There is also the plm() function in the "plm" package which provides fixed
effects models (among many other models for panel data).
However, if I recall correctly, it internally employs the full regressor
matrix, not the demeaned one. More details are explained in the vignette,
vignette("plm", package = "plm")
> Does Bill Venables describe how to do extend the lm() function? I googled
> "course notes on advanced programming Venables", but did not find it. Do
> you have a better link? (hopefully, this is a short explanation---I know
> the algorithm. I want to learn how to coax it into an lm statement.)
> Ivo Welch (ivo.welch at brown.edu, ivo.welch at gmail.com)
> I would just have used lm(y~x+z+factor(firmid)). Admittedly, you get a
>> whole bunch of uninteresting coefficients in the output, but it's not that
>> hard to subset them out.
>> There are two implementation of this in Bill Venables' course notes on
>> advanced programming. I think they are also in 'S Programming', but I can't
>> find my copy right now. These were motivated by computational problems: the
>> full design matrix for the linear model was too large for memory at the time
>> (last century).
>> As a final note, I would strongly discourage
>> r= lm( y ~ x + z, fixed.effects=firmid )
>> as a specification, and would argue for
>> r= lm( y ~ x + z, fixed.effects=~firmid )
>> I think the ability to have some subset of the arguments in a modelling
>> call silently treated as formulas was a bad decision, although it must have
>> looked user-friendly at the time.
>> Thomas Lumley Assoc. Professor, Biostatistics
>> tlumley at u.washington.edu University of Washington, Seattle
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