Version info: Code for this page has been updated to Stata 12.
Page 120 shows how to compare treatment means following a one way anova. First, let's run the basic anova.
use http://www.ats.ucla.edu/stat/stata/examples/da/chap2, clear
anova score a
Number of obs = 15 R-squared = 0.5526
Root MSE = 3.76386 Adj R-squared = 0.4781
Source | Partial SS df MS F Prob > F
-----------+----------------------------------------------------
Model | 210.00 2 105.00 7.41 0.0080
|
a | 210.00 2 105.00 7.41 0.0080
|
Residual | 170.00 12 14.1666667
-----------+----------------------------------------------------
Total | 380.00 14 27.1428571
Page 120 shows a comparison of group 1 versus 2 and 3, and a second comparison of group 2 versus 3. We will use the contrast command with user defined weights.
contrast {a 1 -.5 -.5}{a 0 1 -1}, effects
Contrasts of marginal linear predictions
Margins : asbalanced
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| df F P>F
-------------+----------------------------------
a |
(1) | 1 13.24 0.0034
(2) | 1 1.59 0.2315
Joint | 2 7.41 0.0080
|
Residual | 12
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| Contrast Std. Err. t P>|t| [95% Conf. Interval]
-------------+----------------------------------------------------------------
a |
(1) | 7.5 2.061553 3.64 0.003 3.008262 11.99174
(2) | -3 2.380476 -1.26 0.232 -8.186612 2.186612
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