Date:         Fri, 17 Mar 2006 12:15:02 -0600
Reply-To:     "Swank, Paul R" <Paul.R.Swank@UTH.TMC.EDU>
Sender:       "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From:         "Swank, Paul R" <Paul.R.Swank@UTH.TMC.EDU>
Subject:      Re: Proc Mixed (of course)
Comments: To: Wendy Buzy <wbuzy@PSYCH.UMARYLAND.EDU>
Content-Type: text/plain; charset="us-ascii"

Hi Wendy,

How are you?

See below.

Paul R. Swank, Ph.D.
Professor, Developmental Pediatrics
Director of Research, Center for Improving the Readiness of Children for
Learning and Education (C.I.R.C.L.E.)
Medical School
UT Health Science Center at Houston

-----Original Message-----
From: SAS(r) Discussion [mailto:SAS-L@LISTSERV.UGA.EDU] On Behalf Of
Wendy Buzy
Sent: Friday, March 17, 2006 11:13 AM
To: SAS-L@LISTSERV.UGA.EDU
Subject: Proc Mixed (of course)

I apologize for sending this question to the list, but I've read many of
the previous posts regarding Proc Mixed and my poor, pea-sized brain is
still struggling.  I'm trained as a clinical psychologist and my
statistical knowledge is somewhat limited, although I'm trying my best
to learn!

I'm trying to analyze data from a task that subjects performed 6 times
-- i.e., at 3 different levels of difficulty (Level) and at 2 different
interstimulus intervals (ISI) within each difficulty level.  Thus, each
subject performed the task at:

* Level=1, ISI=3200
* Level=1, ISI=2800
* Level=2, ISI=3200
* Level=2, ISI=2800
* Level=3, ISI=3200
* Level=3, ISI=2800

in that order, on the same day.  Some subjects have missing data, which
is why we're trying to use Proc Mixed rather than Proc GLM.

There are two groups of subjects (ADHD vs. NC).

So far, we've struggled with when/whether to use the RANDOM statement
versus the REPEATED statement, and also with which covariance
structure(s) to use.  We've come up with the following code:


proc sort data=usethis; by subject level descending isi; run; proc mixed
data=usethis order=DATA;
  class group level isi subject;
  model accuracy = group | level | isi;
  repeated level isi / type=un@cs subject=subject; run;


My questions are as follows:

1. Are we using the REPEATED statement correctly and/or should we be
using the RANDOM statement (instead)?
************************************************************************
***************************
As long as you don't wish to paramterize level with random coefficients
(like a growth curve model)
then you are alright.
************************************************************************
****************************

2. It seems that because there are only two levels of ISI, it doesn't
matter whether we specify "type=un@cs" or "type=un@ar(1)", etc.  Is that
correct?
************************************************************************
****************************
That's correct.
************************************************************************
****************************

3. Is it appropriate to be using an unstructured covariance structure?

************************************************************************
****************************
The only direct product forms supported have un for the first matrix.
************************************************************************
****************************

4. I would like to use intelligence (IQ) as a covariate.  Is it correct
to simply add IQ to the model statement (but not the class statement)?

************************************************************************
****************************
That's all. You may wish to check if the covariate by group interaction
is significant though.
************************************************************************
****************************

Thanks in advance for any help you can provide, especially if it can be
phrased using language I might understand.  :)

Regards,
Wendy