[Rasch] Modeling response time

Stuart Luppescu lupp at uchicago.edu
Fri Feb 3 06:55:29 AEDT 2017

On Tue, 2017-01-31 at 19:00 -0200, Ricardo Primi wrote:
> Hi Stuart
> I guess that you don’t want to model interaction . You want to model
> main effects of item and person to and then inspect the residuals
> trying to see who is far off the predicted RT from 
> this model considering additive main effects  right? Besides
> including interaction make this model  non-identified because you
> have an interaction parameter for each combination of personX item
> (minos 1)
> Usually IRT models are just main effects models (person as a random
> variable and item as fixed or sometimes random variables).
> Interactions are used to assess DIF. In this case you have several
> items from the same group and this makes the identification of
> interaction possible (you have the same interaction parameter for th
> items of the same group)
> So i think the model would be :
> lmer(log(answer_time) ~ qid + (1|examinee), data=test.DF, REML=FALSE)
> And then examine the residuals after fitting this model

This worked great! Thanks very much, Ricardo.
Stuart Luppescu
Chief Psychometrician (ret.)
UChicago Consortium on School Research

lupp at uchicago.edu

More information about the Rasch mailing list