Hi
I am running the following PROC MIXED precedure to examine difference between 2 groups with the difference between another 2 groups.
PROC MIXED;
CLASS group;
MODEL Y = group + x1 + x2;
RUN;
Categorical variable "group" has four levels: 1, 2, 3, 4.
I would like to compare the difference in Y between group 1 and 2 to the difference in Y between group 3 and 4. How can I do it?
Thanks,
Do not use plus signs in the MODEL statement. For the comparison, use the CONTRAST statement:
CONTRAST 'Difference of differences 1 - 2 - (3 - 4)' group 1 -1 -1 1;
PG
Do not use plus signs in the MODEL statement. For the comparison, use the CONTRAST statement:
CONTRAST 'Difference of differences 1 - 2 - (3 - 4)' group 1 -1 -1 1;
PG
Thank for answering my question. It is very helpful.
I meet another challenge:
Another confounder (e.g., x3) in group 1 and group 3 is physiological higher than in group 2 and group 4. So I cannot include x3 as a independent variable in the model since putting x3 in the model will allow x3 equal in the four groups. To make the comparison of Y between (1-2) - (3-4) reasonable, I have to allow x3 comparable between group 1 and group3 and between group 2 and group 4.
How can I achieve it?
Thanks,
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