[Rasch] Estimation with ICC

Anthony James luckyantonio2003 at yahoo.com
Tue Dec 16 07:27:14 EST 2008

Hi folks,
In fitting the Rasch model we need to know the ability estimates a priori and make j ability groups. On the y axis we represent the proportion of correct responses in each ability group. Then arbitrary values as item measures are suggested and the probabilities of correct replies for every ability level are estimated using the logistic function. These probabilities are then compared with the proportions correct for each ability level. If they don’t agree the value of the item measure is adjusted until they agree. 
I was always under the impression that both ability and item measures can be read off from the curves. But it seems that the ICC has only a cosmetic or a didactic role in Rasch and IRT and everything is done with the algebra. Without ability estimates drawing ICCs is not possible. Beside an ICC is drawn for an item and estimating person abilities which are based on taking several items is not possible. 
Id be grateful for some explanations on the importance of the curves and the curve fitting endeavor in Rasch analysis, since I cannot appreciate the centrality of the curve and curve fitting.

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