On 4/2/09, Eva Poen <[email protected]> wrote:
> - Although it appears to be a very elegant solution, some people say
> that FLM is not well suited for problems with a lot of zeros or ones;
> for example, Maarten Buis said so in this post (but didn't provide a
> reference): http://www.stata.com/statalist/archive/2007-07/msg00786.html
> If someone knows any references where this is discussed, I'd be
> grateful to receive them.
If you have figured out -gllamm-, then you might be able to use it to
set up a mixture/zero-inflation model with two-point distribution of
the latent variable, using -ip(f) nip(2)- options for the relevant
part of the model. I would probably be more convinced if you had full
panels that consist of zeroes, and other panels that have a mixture of
0s and non-zeroes, rather than each panel having 5 zeroes and one or
two non-zeroes, since zero-inflation models are essentially stating
that an individual is either in "don't-do-it" class with zero outcome,
or "do-it-sometimes" class with zero outcomes coming in a random way
along with non-zeroes. See -zip- for a canned routine doing this in
official Stata.
And btw it might be worth looking at -xt[me]poisson- if your data are
integers. See if interpreting your dependent variable as a count is at
least an approximately reasonable interpretation in your application.
> - I am getting sensible estimates for the random effects with the
> tobit approach, and not so sensible ones with FLM. In fact, FLM
> estimates two of the three to be zero. Is this a sign of my model
> being incorrectly specified, or could it be a sign of FLM not handling
> the zeros and ones very well?
As far as I know (and you should not over-rely on this :)), it is the
tobit model that is usually behaving in a weird way, as it is quite
fragile to the violations of normality assumptions. So I probably
wouldn't put too much value into this kind of comparison; in all
likelihood, BOTH models are misspecified, you just need to find the
one that is more reasonable than the other :)). Dig for George Box's
quote on this!
--
Stas Kolenikov, also found at http://stas.kolenikov.name
Small print: I use this email account for mailing lists only.
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