[Rasch] Two Groups, Pre & Post

Petroski, Greg PetroskiG at health.missouri.edu
Fri May 5 08:04:39 EST 2006

I will have data from a typical design -- two groups (standard treatment
and an enhanced treatment) observed at two time points (admission and
discharge from treatment).  The primary research question concerns group
differences, pre to post, but within group changes will be of interest

The outcome is an ordinal scaled instrument that has been calibrated
with the Partial Credit in other studies.  I would like to apply Rasch
scoring for the obvious reasons and use each subjects' Rasch logits in
place the ordinal scores for subsequent analysis.   

Assume that the two groups do not differ at admission with respect to
the outcome of interest.  I can see at least two ways to apply Rasch
scoring and would like the advice of those with more experience with
Rasch methods.  

1) Pool all the data (both groups, pre and post) and do a single
calibration.  Each subject is used twice in this scheme (pre and post)
but as we are only scoring the tool in this step and not testing any
hypotheses I do not see that the lack of independence is an issue.  

2) Pool both groups at baseline, calibrate.  Also calibrate both groups'
discharge scores separately and use equating methods to place pre and
post Rasch scores on a common scale prior to analysis.    It seems like
the equating step introduces some error that we do not have in #1 above.

Comments on the merits of 1 vs 2, or other approaches will be
appreciated.  Any citations that use Rasch methods for this type of
design would also be of interest.

Greg Petroski, Ph.D.
Health Management and Informatics &
Office of Medical Research/Biostatistics
137 Hadley Hall (DC 018)
University of Missouri - Columbia,
Columbia, Mo.  65212
ph:  (573)882-3373
fax: (573)882-7909


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