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

[Richard Williams <richardwilliams.ndu@gmail.com>]

At 09:19 PM 5/25/2010, Michael N. Mitchell wrote:
> Dear Richard
>
>   I think I need to use my glasses!
>
>   Yes, Richard, you are exactly on target. It relates to the use of 
> -#- instead of -##- .
>   My previous answer is still true, in the sense that when you do 
> a#b and b#a, that you get a different reference "cell", and thus it 
> is re-scrambling the coding.
I agree with everything you say.  But, it still isn't clear to me why 
it should make a difference whether you use b1.ra#b0.dm versus 
b0.dm#b1.ra.  Tweaking your example, the following yield identical or 
equivalent results for regress but not for poisson:
sysuse auto, clear
generate bigtrunk = trunk > 15
generate biglen = length > 190
regress mpg bigtrunk#biglen
regress mpg b1.bigtrunk#b0.biglen
regress mpg b0.biglen#b1.bigtrunk
poisson mpg bigtrunk#biglen, nolog
poisson mpg b1.bigtrunk#b0.biglen, nolog
poisson mpg b0.biglen#b1.bigtrunk, nolog

Use of ## in the last 2 commands avoids the problem, but why is there 
a problem in the first place?
-------------------------------------------
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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