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AR(1) Covariance Matrix Structure for Time points that are not evenly spaced?

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AR(1) Covariance Matrix Structure for Time points that are not evenly spaced?

Hi- I am a student analyzing data in SAS Studio. My data set has repeated measurements on subjects who have been randomly assigned to either a treatment or a placebo group.

 

The beginning of my data set looks like this:

subject trt sex age visit time visual
1 Active F 71 1 0 59
1 Active F 71 2 4 55
1 Active F 71 3 12 45

 

 I have measurements of my response variable: visual, at 5 timepoints: baseline (0), 4, 12, 24, and 52 weeks.

 

I wrote the following code to analyze my data:

*proc mixed analysis: repeated measures  with AR(1) errors;

proc mixed covtest data=MDwithMissing;

class trt subject time;

model visual=trt time trt*time/ddfm=kr;

random subject(trt);

repeated time/sub=subject(trt) type=ar(1) r rcorr;

lsmeans time time*trt;

run;

 

Is it appropriate to use an ar(1) covariance error structure to model the covariance between time points even though my time points are not all an equal distance from each other?

 

Thanks for your help!

Sheila

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