[Rasch] Comparison Rasch 3-parm IRT
rprimi at mac.com
Fri Sep 13 11:50:12 EST 2013
Dear List members
I have a question about comparison of Rasch model. vs IRT 3 param model. I am working in the development of a national test for entering teaching profession in Brazil. I recently analyzed data of a pre-test item bank and calibrated the items/persons using the Rasch model (centering the scale on the persons side: UPMEAN = 0) and also in XCALIBRE using the 3-parameter model (MML with a Bayesian prior of mean 0 and SD 1, that is, also centered on persons). So I had two estimates for each one of the same group or persons. Unidimensinality was ok (done using parallel analysis in psych). Item had generally lower discrimination and c around .20. M and SD of both measures and theta estimates were very similar (0 and .79). Correlation between the two was very high: .97. I then compared information functions from the two calibrations. I transformed information > standard errors > local reliability – LR (Daniel, 1999): LR = 1 - ( (se ** 2) / variance of theta ). I saw that the rash local reliabilities (information) for the persons measures were higher (M=.70) than the 3-parm (M=.56). My question: Is this possible ? Is it because of a and c ? Is there some error in this procedure that I missed ?
I attached the plot of the LR. I computed model and real local reliabilities for the rasch measures and both are higher then IRT counterparts ..
Thank you in advance from your inputs ..
USF, Brazil (www.labape.com.br)
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