### Stata Textbook Examples Design and Analysis by Geoffrey Keppel Chapter 7: Analysis of Trend

Version info: Code for this page has been updated to Stata 12.

In this chapter, Keppel shows how to do trend analysis. Let's start by running the overall anova as shown in table 7-1 on page 143.
use http://www.ats.ucla.edu/stat/stata/examples/da/chap7, clear
On page 145, Keppel shows a test of linear trend, and a test of quadratic trend on page 148, and a test of cubic trend on page 150. We will use the contrast command with the p. operator to test these trends.
anova numcorr a

Number of obs =      80     R-squared     =  0.2973
Root MSE      = 4.50424     Adj R-squared =  0.2695

Source |  Partial SS    df       MS           F     Prob > F
-----------+----------------------------------------------------
Model |       652.3     3  217.433333      10.72     0.0000
|
a |       652.3     3  217.433333      10.72     0.0000
|
Residual |      1541.9    76  20.2881579
-----------+----------------------------------------------------
Total |      2194.2    79  27.7746835

contrast p.a, effects

Contrasts of marginal linear predictions

Margins      : asbalanced

------------------------------------------------
|         df           F        P>F
-------------+----------------------------------
a |
(linear)  |          1       12.62     0.0007
(quadratic)  |          1       19.52     0.0000
(cubic)  |          1        0.01     0.9119
Joint  |          3       10.72     0.0000
|
Residual |         76
------------------------------------------------

------------------------------------------------------------------------------
|   Contrast   Std. Err.      t    P>|t|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
a |
(linear)  |   1.788854   .5035891     3.55   0.001     .7858698    2.791839
(quadratic)  |     -2.225   .5035891    -4.42   0.000    -3.227985   -1.222015
(cubic)  |   .0559017   .5035891     0.11   0.912    -.9470829    1.058886
------------------------------------------------------------------------------

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