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Re: st: How to constraint the sum of the coefficients to zero for a factor variable.
From
Richard Williams <[email protected]>
To
[email protected], [email protected]
Subject
Re: st: How to constraint the sum of the coefficients to zero for a factor variable.
Date
Fri, 05 Aug 2011 10:41:21 -0500
At 07:19 AM 8/5/2011, Maarten Buis wrote:
On Fri, Aug 5, 2011 at 1:28 PM, Sam Brilleman wrote:
> Does anyone know how to change the constraint on the coefficients
for 'var4'
> so that the sum of the coefficients is zero (i.e. sum(b4i) = 0) rather than
> treating one of the factor levels as a reference category (i.e. b4i = 0 for
> some given i). Note that the latter is the standard parameter constraint
> used by STATA's estimation commands.
I don't know of a way to do this in Stata's factor variable notation.
You can however create the necesary variables using -xi3-, where they
are called "effect coding", see -findit xi3-.
Hope this helps,
Maarten
I can't say that I understand it yet, but does the -contrast- command
in Stata 12 offer a way to deal with this? The help says
"contrast tests linear hypotheses and forms contrasts involving
factor variables and their interactions from the most recently fit
model. The tests include ANOVA-style tests of main effects, simple
effects, interactions, and nested effects. contrast can use named
contrasts to decompose these effects into comparisons against
reference categories, comparisons of adjacent levels, comparisons
against the grand mean, orthogonal polynomials, and such. Custom
contrasts may also be specified."
-------------------------------------------
Richard Williams, Notre Dame Dept of Sociology
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EMAIL: [email protected]
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