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st: Re: Anova and contrasts


From   "Janet Pol" <[email protected]>
To   [email protected]
Subject   st: Re: Anova and contrasts
Date   Mon, 11 Dec 2006 14:45:56 +0000

I am having a problem with contrast following this anova:

.. anova eff system material immerse system*material system*immerse material*immerse system*material*immerse if material>0

Number of obs = 180 R-squared = 0.5815
Root MSE = .036934 Adj R-squared = 0.5376

Source | Partial SS df MS F Prob > F
------------------------+----------------------------------------------------
Model | .307109453 17 .018065262 13.24 0.0000
|
system | .027877154 4 .006969288 5.11 0.0007
material | .195840007 1 .195840007 143.56 0.0000
immerse | .000765629 1 .000765629 0.56 0.4548
system*material | .02563412 4 .00640853 4.70 0.0013
system*immerse | .02743688 3 .009145627 6.70 0.0003
material*immerse | .013875634 1 .013875634 10.17 0.0017
system*material*immerse | .025326868 3 .008442289 6.19 0.0005
|
Residual | .22099014 162 .001364137
------------------------+----------------------------------------------------
Total | .528099592 179 .002950277

I am interested in looking at the system*material interactions in more detail. I have carried out "all possible" comparisons using:
.. test _coef[system[1]*material[1]]=_coef[system[1]*material[2]]
etc.
It occurs to me that I should setup a contrast matrix test this using the mtestand allow for multiple testing. The problem is that the design matrix has 54 columns. Do I need to set up this matrix for the 4 comparisons that I need or is there a more efficient (or simpler) approach.
Many thanks.
Janet

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