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# How to statistically comapre difference between two groups with the difference between another two groups

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,

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‎06-22-2013 12:37 PM
Posts: 5,543

## Re: How to statistically comapre difference between two groups with the difference between another two groups

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

PG

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Solution
‎06-22-2013 12:37 PM
Posts: 5,543

## Re: How to statistically comapre difference between two groups with the difference between another two groups

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

PG
Contributor
Posts: 21

## Re: How to statistically comapre difference between two groups with the difference between another two groups

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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