Hi Alex,

Just to clarify in case I wasn't clear - I'm quite comfortable with running these kinds of repeated measures mixed models in SPSS. It's just the additional 'period' difference in variances that I haven't been able to work out how to define.

Re your specific questions, the data file is indeed as you illustrate however all rows with time=1,2 or 3 will have period=1. Subjects underwent the four conditions in random order. During each condition 23 repeated measures were taken.

Thanks again, Kylie.

-----Original Message----- From: SPSSX(r) Discussion [mailto:SPSSX-L@LISTSERV.UGA.EDU] On Behalf Of Reutter, Alex Sent: Friday, 11 July 2008 12:54 pm To: SPSSX-L@LISTSERV.UGA.EDU Subject: Re: variance structures in MIXED

What's the structure of your data? Something like:

subject time condition period response 1 1 1 1 ### 1 1 2 1 ### 1 1 3 1 ### 1 2 1 2 ### 1 2 2 2 ### 1 2 3 2 ### 1 3 1 2 ### ... 1 23 1 2 ### 1 23 2 2 ### 1 23 3 2 ### 2 1 1 1 ### 2 1 2 1 ### 2 1 3 1 ### ...

Or perhaps time and condition reversed? Precisely how were the experimental conditions administered (i.e., were subjects assigned a condition and then followed for 23 time periods, then assigned another condition and followed for 23 time periods, then the third condition and followed for the final 23 time periods?)

Alex

-----Original Message----- From: SPSSX(r) Discussion [mailto:SPSSX-L@LISTSERV.UGA.EDU] On Behalf Of Kylie Lange Sent: Thursday, July 10, 2008 8:42 PM To: SPSSX-L@LISTSERV.UGA.EDU Subject: variance structures in MIXED

Hi all,

I am trying to replicate some analyses that were originally done in SAS. I do not have the code that was used, but do have the analysts writeup of the analysis method and results. I am trying to confirm if the variance structure they used is also possible in SPSS.

It is a doubly repeated measures design - multiple time points (23) per experimental condition (3) per subject. The model includes fixed effects for time (entered as a factor), condition and the time*condition interaction. The description of the previous analysis includes this statement regarding the variances:

"Due to different variances in the outcome across time, variances were estimated separately for baseline measures (the first three time points), and then for all measures after."

From the output provided I can see that a variable was created ('period') that equalled 1 for the observations at the three baseline timepoints, and 2 for all the others. The 'Covariance Parameter Estimates' table from SAS includes 5 parameters:

Cov Parm Subject Group

Intercept id period 1

Intercept id period 2

time id period 1

time id period 2

AR(1) id

Error

Can anyone suggest how to define this kind of model in SPSS' MIXED procedure?

Thanks,

Kylie.

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