does anyone know if when i check for an interaction in the ancova model and i get a significant interaction, can i use the results i recived ?
if not what do i do ???
thanks
CL
The presence of an interaction means that the effect of your covariate is not the same in every group (class variable). A comparison of the dependent variable will thus depend on the value of your covariate. What you can do is look at the ancova plot and compare your groups at a specific value of your covariate, as in this example (modified from SAS doc) :
data DrugTest;
input Drug $ PreTreatment PostTreatment @@;
datalines;
A 11 6 A 8 0 A 5 2 A 14 8 A 19 11
A 6 4 A 10 13 A 6 1 A 11 8 A 3 0
D 6 0 D 6 2 D 7 3 D 8 1 D 18 18
D 8 4 D 19 14 D 8 9 D 5 1 D 15 9
F 16 13 F 13 10 F 11 18 F 9 5 F 21 23
F 16 12 F 12 5 F 12 16 F 7 1 F 12 20
;
proc glm data=DrugTest plots(only)=(ancovaplot(clm));
class Drug;
model PostTreatment = Drug|PreTreatment;
lsmeans Drug / at PreTreatment=15 pdiff;
run;
PG
The presence of an interaction means that the effect of your covariate is not the same in every group (class variable). A comparison of the dependent variable will thus depend on the value of your covariate. What you can do is look at the ancova plot and compare your groups at a specific value of your covariate, as in this example (modified from SAS doc) :
data DrugTest;
input Drug $ PreTreatment PostTreatment @@;
datalines;
A 11 6 A 8 0 A 5 2 A 14 8 A 19 11
A 6 4 A 10 13 A 6 1 A 11 8 A 3 0
D 6 0 D 6 2 D 7 3 D 8 1 D 18 18
D 8 4 D 19 14 D 8 9 D 5 1 D 15 9
F 16 13 F 13 10 F 11 18 F 9 5 F 21 23
F 16 12 F 12 5 F 12 16 F 7 1 F 12 20
;
proc glm data=DrugTest plots(only)=(ancovaplot(clm));
class Drug;
model PostTreatment = Drug|PreTreatment;
lsmeans Drug / at PreTreatment=15 pdiff;
run;
PG
In your attached example the interaction not significant.
I would like to know if it is a problem to use the ancova results when i get a significant interaction (is it an assumption that the slopes must be the same in order to using the ancova results?)?
thanks
CL
You cannot fit an ANCOVA model (model PostTreatment = Drug PreTreatment;) when the interaction is significant. My example shows you what can be done when the interaction IS significant. - PG
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