[R-sig-ME] longitudinal with 2 time points

[John Maindonald]

All these are possibilities, except maybe making baseline measurement
a random factor.  This would make sense only if data divide into groups,
and you want the baseline effect to vary randomly from group to group.  
That may limit your ability to estimate parameters that are of interest.
In most circumstances that I am familiar with, it makes better sense to 
treat baseline effect as fixed.

John.

John Maindonald             email: john.maindonald at anu.edu.au
phone : +61 2 (6125)3473    fax  : +61 2(6125)5549
Centre for Mathematics & Its Applications, Room 1194,
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Australian National University, Canberra ACT 0200.
http://www.maths.anu.edu.au/~johnm

On 11/08/2010, at 8:11 AM, array chip wrote:

> Hi, I am wondering if it is still meaningful to run a mixed model if a 
> longitudinal dataset has only 2 time points (baseline and week 4)? Would it be 
> more appropriate to simply take the difference between the 2 time points and run 
> ANOVA (ANCOVA) on the difference? what about still running mixed model on the 
> difference of the 2 time points, but adding baseline measurement as a random 
> factor?
> 
> Thanks for sharing your thoughts.
> 
> John
> 
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