Re: st: Understanding Factor variables - is order significant ?

["Michael N. Mitchell" <Michael.Norman.Mitchell@gmail.com>]

Dear Richard

  That is an excellent summary, and I agree completely. Thanks for patiently walking me 
through this! I hope others have enjoyed this :) .

Michael N. Mitchell
Data Management Using Stata      - http://www.stata.com/bookstore/dmus.html
A Visual Guide to Stata Graphics - http://www.stata.com/bookstore/vgsg.html
Stata tidbit of the week         - http://www.MichaelNormanMitchell.com



On 2010-05-25 9.22 PM, Richard Williams wrote:
> > This is a perplexing state of affairs! I don't know how to explain this!
> >
> > I hope someone can help explain!
>
> Michael, I am relieved that someone else is confused! Tweaking your code
> a bit,
>
> use http://www.ats.ucla.edu/stat/stata/dae/poissonreg, clear
> gen himath = math > 50
> poisson daysabs himath#male, nolog
> poisson daysabs ib0.male#ib1.himath, nolog
> poisson daysabs ib1.himath#ib0.male, nolog
> poisson daysabs ib0.male##ib1.himath, nolog
> poisson daysabs ib1.himath##ib0.male, nolog
>
> The first 2 models give the correct result -- LR Chi-square = 202.49.
> The 3rd gives the incorrect result of LR chi-square = 144.99. The last 2
> commands, using ## instead of #, also give the correct results.
>
> In the few examples we've tried where we knew the correct answer, it
> looks like interactions of the form b0.x1#b1.x2 worked fine but
> b1.x2#b0.x1 caused problems. Further, plain old regress seems to work
> fine regardless of how you do the interactions but ml techniques like
> poisson and ologit have problems.
>
> I strongly suspect there is some sort of bug here. But if not, maybe
> we'll get a fascinating explanation that will greatly add to our
> understanding of interaction effects!
>
> -------------------------------------------
> Richard Williams, Notre Dame Dept of Sociology
> OFFICE: (574)631-6668, (574)631-6463
> HOME: (574)289-5227
> EMAIL: Richard.A.Williams.5@ND.Edu
> WWW: http://www.nd.edu/~rwilliam
>
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