review and some dummies

1. SAS tutorial 0524

2. proc anova;
class A B S;
model y=S|A|B;
test h=A e= S*A;
run;
The same as glm （但是比較沒用...）

3. Example
title 'Unbalanced Two-Way 2 1 28 . 23 34 42 13
Analysis of Variance'; 2 2 . 34 33 31 . 36
data a; 2 3 3 26 28 32 4 16
input drug disease @; 3 1 . . 1 29 . 19
do i=1 to 6; 3 2 . 11 9 7 1 -6
input y @; 3 3 21 1 . 9 3 .
output; 4 1 24 . 9 22 -2 15
end; 4 2 27 12 12 -5 16 15
datalines; 4 3 22 7 25 5 12 .
1 1 42 44 36 13 19 22 ;
1 2 33 . 26 . 33 21
1 3 31 -3 . 25 25 24

4. 4x3 2-way

5. code
ods graphics on;
proc glm;
class drug disease;
model y=drug disease drug*disease /ss3;
contrast 'drug 1 vs 2' drug 1 -1 0 0;
contrast 'drug 1 vs 3' drug 1 0 -1 0;
contrast 'drug 1 vs 4' drug 1 0 0 -1;
lsmeans drug/ pdiff=all adjust=tukey;
run;
ods graphics off;

6. Result

7. Interaction plot

10. Dummy variable
• Category variable
• Set reference group
• Set N-1 d.variables:
– 1 if xij belongs to the group specified
– 0 if xij belongs to other groups
• E.g. variable drug in the example:
– 0 0 0 => drug=1
– 1 0 0 => drug=2
– 0 1 0 => drug=3
– 0 0 1 => drug=4

11. Data

12. Result (is the same)

13. Why use d.v. coding?
http://www.sas.com/offices/NA/canada/downloads/presen
tations/VancouverMay11/Variable.pdf

15. If you cannot run contrast
analysis──
• Check coefficients
• Separate main effects and interactions into
different models.
• e.g. 2-way ANOVA with interaction:
– model y = A B;
(contrasts for main effects)
– model y = A*B;
(contrasts for interaction)