I have estimated a Partial Credit Model in Stata following Zheng, X. and Rabe-Hesketh, S. (2007) "Estimating parameters of dichotomous and ordinal item response models using gllamm".
I'd like to compute Item Information Curves but I can't figure out how to do it from what I found in Google or in other StackExchange posts.
a) In Ayala's "the theory and practice of IRT" book, pg 200, it is explained that the item information function in the PCM model is obtained as the weighted sum of category information functions. But in the formula for item information function I don't see where do I sum across item category options.
b) Is the item information formula for the Partial Credit model valid for the 2-paramenter version (Generalized Partial Credit Model)? If not, what is the formula for this?
Any guidance would be much appreciated.