Date:         Wed, 28 Dec 2011 15:01:44 -0500
Reply-To:     Rich Ulrich <rich-ulrich@live.com>
Sender:       "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From:         Rich Ulrich <rich-ulrich@live.com>
Subject:      Re: GLM repeated measures concern/question
Comments: To: eyeman03@hotmail.com
In-Reply-To:  <1325024970053-5104022.post@n5.nabble.com>
Content-Type: multipart/alternative;


Do you really  have reason to throw out the assumption of
equal spacing?

The set sizes (1,8,32) are obviously non-linear in spacing
when you consider simple counts, but the dimension that 
matters is the dimension of impact on the measured outcome.  
Thus, "equal intervals" for a drug dosage will sometimes be 
in log-units, rather than raw units - as a simple example.

It is my experience that experienced researchers pick 
"unequal intervals" in time, in setting up repeated measures, 
in such a fashion to approximate equal intervals in outcome.  
I've usually had "time" for the repeated measures I've tested. 
And it is a mistake not to take advantage of the intelligent
design when doing the analyses.

It is my observation that nominally unequal numeric intervals,
whether they are time or design elements like yours, are 
usually (similarly) constructed with the intent of producing
equal intervals of outcome/ response.  So you might try the
simple analysis, assuming equal intervals, and see if fits.

-- 
Rich Ulrich 

> Date: Tue, 27 Dec 2011 14:29:30 -0800
> From: eyeman03@hotmail.com
> Subject: GLM repeated measures concern/question
> To: SPSSX-L@LISTSERV.UGA.EDU
> 
> Hola,
> 
> We have a potential concern with analyzing our data:
> 
> *Quick Backgroud:*  Research Design is 2x2 Mixed Model involving the effects
> of central blind spots on Visual Search.  The subjects are searching for a
> target (O) embedded among distracters (C’s).  The set sizes we have used are
> 1, 8, and 32 (1 stimuli on screen =1set size; 8 stimuli on screen= 8 set
> size and so forth).
> 
> *Problem*: The potential concern is the set sizes (predictor variables) are
> NON-LINEAR in spacing (unequal). Our plan is to use GLM Repeated Measures to
> analyze the data, but we’re wondering if this GLM model can be used with the
> set size non-linearity…………another words does the GLM model implicitly assume
> linear spacing with its analyses?
> 
> If yes, what are the potential solutions?
[snip]
                                          

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