Post Hoc Tests - Agricultural Statistical Data Analysis Using Stata - page 148

Table 8.1 Orthogonal coefficients for planned comparisons

CONTRAST CONTROL NH 4 SO 3 GREEN LEAF NH 4 SO 3 + GREEN LEAF

Control versus fertilization 3 -1 -1 -1

NH 4 SO 4 versus green leaf 0 1 -1 0

NH 4 SO 4 and green leaf versus
NH 4 SO 4 + green leaf

0 1 1 -2

test 3 * 1.trt-2.trt-3.trt-4.trt=0

( 1) 3*1b.trt - 2.trt - 3.trt - 4.trt = 0

F( 1, 9) = 82.57
Prob > F = 0.0000

In addition to the test command, there is the contrast command
that can be used. Enter the following and see the results:

contrast { trt 3 -1 -1 -1 }

Contrasts of marginal linear predictions

Margins : asbalanced

------------------------------------------------
| df F P>F
-------------+----------------------------------
trt | 1 82.57 0.0000
|
Residual | 9
------------------------------------------------

--------------------------------------------------------------
| Contrast Std. Err. [95% Conf. Interval]
-------------+------------------------------------------------
trt |
(1) | -15.475 1.703 -19.32745 -11.62255
--------------------------------------------------------------

There is slightly more information shown with the contrast com-
mand, but the saved scalars from the test command are more useful.
Stata will evaluate any algebraic expression before calculating the
test command. The additional comparisons are