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Re: st: Wald test limit?


From   Maarten buis <maartenbuis@yahoo.co.uk>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Wald test limit?
Date   Mon, 4 Feb 2008 23:06:09 +0000 (GMT)

The fact that it is a round number is not suspicious, it should be an
integer. One possible reason why it has less degrees of freedom than
you expect is that Stata might have dropped some variables due to
multicolinearity (this is a very real possibility when dealing with
such a large number of predictors).

Hope this helps,
Maarten

--- Keith Dear <keith.dear@anu.edu.au> wrote:

> A student of mine is fitting large nbreg models. One such 
> successfully estimates 262 parameters (not including alpha) but the 
> Wald test reports only 256df - anyone know why? Of course 256 is a 
> suspiciously round number so probably it's just a limit, but there's 
> a further wrinkle:
> 
> This model was arrived at using stepwise, then fitted anew by listing
> 
> the terms that stepwise kept (thus keeping a few more rows of data). 
> The two logs show exactly the same 262 parameters (with slightly 
> different estimates), yet the stepwise ends with a Wald test on 255 
> df, not even 256 - why?
> 
> * code for the two fits ($BASE lists many terms):
> xi: stepwise, pr(0.05) lockterm1: nbreg rrv_count ($BASE)
> (i.fin_year)
> (NDVI_Q0 NDVI_Q1 NDVI_Q2 NDVI_Q3)
> (NDVImax_Q0 NDVImax_Q1 NDVImax_Q2 NDVImax_Q3)
> (NDVImin_Q0 NDVImin_Q1 NDVImin_Q2 NDVImin_Q3)
> (igreys_Q0 igreys_Q1 igreys_Q2 igreys_Q3 noigreys_Q0 noigreys_Q1 
> noigreys_Q2 noigreys_Q3)
> (ireds_Q0 ireds_Q1 ireds_Q2 ireds_Q3 noireds_Q0 noireds_Q1 noireds_Q2
> 
> noireds_Q3),
> exposure(person_days) cl(pcode)
> 
> * result: stepwise drops all three (NDVI) sets but keeps the (greys)
> and (reds)
> * therefore augment $BASE to include all (greys) and (reds):
> glo BASE $BASE igreys_Q0 igreys_Q1 ... noireds_Q3
> xi: nbreg rrv_count (i.fin_year) $BASE, exposure(person_days)
> cl(pcode)
> 
> *
> *   For searches and help try:
> *   http://www.stata.com/support/faqs/res/findit.html
> *   http://www.stata.com/support/statalist/faq
> *   http://www.ats.ucla.edu/stat/stata/
> 


-----------------------------------------
Maarten L. Buis
Department of Social Research Methodology
Vrije Universiteit Amsterdam
Boelelaan 1081
1081 HV Amsterdam
The Netherlands

visiting address:
Buitenveldertselaan 3 (Metropolitan), room Z434

+31 20 5986715

http://home.fsw.vu.nl/m.buis/
-----------------------------------------


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