Discrimination parameters in two-parameter model from IRT are usually considered item parameters.
But I've come to doubt it. Think about psychophysics; for example, detecting luminance. I don't think anyone would say that discrimination (i.e., slope) parameter applies to luminance itself. It comes from human perception, so I figure it's a person parameter and it could be different for each person.
What about test situation? Is it without any doubt that the discrimination parameter is considered as an item parameter? I doubt it is, especially given the fact that there is room for differential item functioning (DIF).
So I'm concerned with the fact there's any theoretical basis for saying that discrimination parameters are item-parameters.
I think I could model person-specific discrimination parameter model, in which item-discrimination parameter is determined by person-specific discrimination parameters in the group. That would justify DIF.
So, my question would sum up like this:
Is that if there's any theoretical/empirical basis for saying that discrimination parameters are item-parameters? How can one be sure of it?
Is there any related reference for person-discrimination parameter model mentioned above?