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# ancova

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

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‎01-06-2013 01:28 PM
Posts: 5,052

## Re: ancova

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

PG

All Replies
Solution
‎01-06-2013 01:28 PM
Posts: 5,052

## Re: ancova

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

PG
Contributor
Posts: 45

## Re: ancova

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

Posts: 5,052

## Re: ancova

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

PG
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