[Rasch] Longitudinal Rasch

Petroski, Greg PetroskiG at health.missouri.edu
Wed Dec 3 05:27:07 EST 2008


I know I asked a related question long ago but after after changing
computers I can not fine the responses.  
 
I want to score an existing scale using the partial credit model and use
these scores to examine change over time.  This is a medical-type
evaluation, is NOT a self-report, and the items are always the same.
The final analysis will be complex with lots of covariates and a nested
data structure, so I would like to score the tool with WinSteps or
ConQuest,  and use the save the trait estimates as the outcome in the
subsequent analysis.  What I am not sure of is how to handle the
repeated measurements on the same people. Each participant will be
evaluated at least four times.  Here is my thinking,
 
1) Ignore the time period and estimate model parameters and "abilities"
just once as though the data was cross-sectional.
 
2) N is large, so estimate the item parameters using the baseline
assessment and treat them as fixed and estimate only abilities for each
subsequent time point.
       Two issues -  Will the software I have do this?  Does one need an
equating step?
 
3) Separate calibrations and use an equating procedure to put abilities
on the same metric.
 
3) Forget Rasch/IRT and use the summated score!
 
Any and all thoughts are welcome.
 
Gregory F. Petroski, PhD
Office of Medical Research
School of Medicine
University of Missouri -Columbia
Columbia, Mo.  65212
 
                      
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