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Version info: Code for this page was tested in 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 create three variables acomp1, acomp2 and acomp3 to test for these three trend components. The variable acomp1 contains the coefficients for linear trend, and acomp2 has the coefficients for quadratic trend, and acomp3 has the coefficients for cubic trend.
generate acomp1=a recode acomp1 1=-3 2=-1 3= 1 4= 3 generate acomp2=a recode acomp2 1= 1 2=-1 3=-1 4= 1 generate acomp3=a recode acomp3 1=-1 2= 3 3=-3 4= 1
Now we enter the variables acomp1, acomp2 and acomp3 into the anova command as continuous variables.
anova numcorr c.acomp1 c.acomp2 c.acomp3
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 | 652.30 3 217.433333 10.72 0.0000
|
acomp1 | 256.00 1 256.00 12.62 0.0007
acomp2 | 396.05 1 396.05 19.52 0.0000
acomp3 | .25 1 .25 0.01 0.9119
|
Residual | 1541.90 76 20.2881579
-----------+----------------------------------------------------
Total | 2194.20 79 27.7746835
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