[Rasch] OPLM - imputable discrimination

Mike Linacre (RMT) rmt at rasch.org
Thu Jul 6 13:20:30 EST 2006


Colleagues:

Has anyone found that imputing item discrimination (so giving the 
'flexibility' of the 2PL model) is useful?  Winsteps already reports an 
item discrimination index. It would be straightforward to make item 
discrimination imputable. Of course, if you estimate => impute => estimate 
=> impute => .... then the process diverges.

MIke Linacre

At 7/5/2006, you wrote:
>Rama, the OPLM is formally the same as the 2PL model. However, item 
>discrimination in the OPLM is treated as an index rather than a parameter. 
>The index is imputed rather than being estimated. So the 'one' in OPLM 
>implies there is still only one item parameter. The weighted score in the 
>2PL model is sufficient for the person parameter but the problem is that 
>the values of discrimination parameters are unknown, meaning the weighted 
>raw score "is not a mere statistic, and hence it is impossible to use CML 
>as an estimation method" (Verhelst & Glas, 1995, p. 217). The approach in 
>the OPLM is to impute the values rather than estimating them, though 
>estimates are still used as starting point. The index is restricted to 
>integer values between 1 and about 15. The OPLM is intended to retain the 
>properties of the Rasch model while giving the 'flexibility' of the 2PL 
>model (stated at the beginning of the manual).

Mike Linacre
Editor, Rasch Measurement Transactions
rmt at rasch.org www.rasch.org/rmt/ Latest RMT: 19:4 Spring


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