So, I've been stuck trying to get item locations from mirt while using GPCM. I know that eRm usually gives you both item location and thresholds, but somehow I haven't been able to find out where is it on mirt. By using:
$ coefG
Are you a member of the Matilda Bay Rasch Forum, I am sure some one on there will know the answer. you will need to be added as a member but its free and the best place to ask IRT and Rasch related questions that I know.
Assuming you define 'location' and 'thresholds' in a way analogous to how it is defined in rating-scale models (Muraki, 1992), what 'coef(modG, IRTpars = TRUE, simplify = TRUE) ' returns you as 'bi' parameters is:
(item_location - item_category_threshold)
Because item_category_thresholds for a given item sums to 0, you can compute item location as a mean of 'bi' parameters and then compute item_category_thresholds by subtracting 'bi' parameters from item location.
Anita, please let me know if that's an open forum.
Tomasz, I'm not sure and I'm sorry as here in Brazil we usually have a poor background in psychometrics. We are trying to get better at it. You really got what I tried to say on the first part. So the solution you suggest is that I can actually report the avarage ability level required for each item. So for example, if I want to provide an analogous location for item 4 in a 5 levels likert scale I can take the average of threshold 1 (0-1), threshold 2 (1-2), threshold 3 (2-3) and threshold 4 (3-4) and report it as item location in a similar way? Or did I get what you tried to say in the wrong way?
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