How to interprete parameters and how to visualize icc plot for each Theta for a more than 2 MIRT model?

[Chengli Yu]

Hi!

I have problems with plotting icc curves (Probability vs Theta) for each theta in a 7 4-catergery(0,1,2,3) items mirt model. 

For example, I used graded itemtype, with two dimensions. The results gave some parameters of a1,a2,d1,d2,d3. Based on the results, should I calculate the probability of each category with following equations?

P0 = 1- 1/(1+exp(a*(b1-theta))) 
P1 = 1/(1+exp(a*(b1-theta))) - 1/(1+exp(a*(b1+b2-theta))) 
P2 = 1/(1+exp(a*(b1+b2-theta))) - 1/(1+exp(a*(b1+b2+b3-theta))) 
P3 = 1/(1+exp(a*(b1+b2+b3-theta)))

Thanks!

[Phil Chalmers]

Is there something wrong with the itemplot() approach when requesting thresholds?

library(mirt)

pmod <- mirt(Science, 1)
itemplot(pmod, 3, type = 'threshold')

Phil

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