PROC MIXED Estimate statements

[lauragrau]

Good morning,

I have a longitudinal dataset with three variables: group, week, and outcome (y). 

Group is binary 0/1.

Week is continuous 1-6.

Outcome is continuous.

I'm running a mixed model, testing whether there was a significant interaction between week and group.

proc mixed data=data;
class id group(ref=first) ;
model y=week group group*week;
random intercept week /sub=id type=un ;
run;

How would I write an estimate statement to test whether there was a significant difference in average y between weeks 1-3 and 4-6 between the two groups? 

[sbxkoenk]

Look at this wealth of info on ESTIMATE and CONTRAST statements in SAS/STAT:

The magical ESTIMATE (and CONTRAST) statements

By Chris Daman on SAS Learning Post April 23, 2012

https://blogs.sas.com/content/sastraining/2012/04/23/the-magical-estimate-and-contrast-statements/

"Easy button" for ESTIMATE statements

By Chris Daman on SAS Learning Post April 25, 2012

https://blogs.sas.com/content/sastraining/2012/04/25/easy-button-for-estimate-statements/

ESTIMATE Statements - the final installment

By Chris Daman on SAS Learning Post May 2, 2012

https://blogs.sas.com/content/sastraining/2012/05/02/estimate-statements-the-final-installment/

How to write CONTRAST and ESTIMATE statements in SAS regression procedures?

By Rick Wicklin on The DO Loop June 6, 2016

https://blogs.sas.com/content/iml/2016/06/06/write-contrast-estimate-statements-sas-regression-proce...

Usage Note 24447: Examples of writing CONTRAST and ESTIMATE statements

https://support.sas.com/kb/24/447.html

Usage Note 67024: Using the ESTIMATE or CONTRAST statement or Margins macro to assess continuous variable effects in interactions and splines

67024 - Using the ESTIMATE or CONTRAST statement or Margins macro to assess continuous variable effe...

Good luck,

Koen

[jiltao]

You treated WEEK as a continuous variable. Then when you say weeks 1-3, do you mean the average for week 1 to week 3? That is, the average value for WEEK is 2? Similarly, weeks 4-6 means the average value for WEEK would be 5? If so, you might see if the following ESTIMATE statement does what you want:

proc mixed data=data;
class id group(ref=first) ;
model y=week group group*week;
random intercept week /sub=id type=un ;
estimate "difference between weeks 1-3 and 4-6 for group 2 versus difference between weeks 1-3 and 4-6 for group 1' group*week -3 3;
run;