The result of including a discrimination parameter is that the slopes of the Item Characteristic Curves are no longer parallel.
Under the Rasch model, for any person, Item 1 is easier than Item 2 which is easier than item 3.
Under the two-parameter IRT model ICCs are not parallel. For a person with ability measure of -1 logit, Item 1 is the easiest item. But for a person with an ability of 2 logits, Item 2 is the easiest item.
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16.1 Exercise
Amend the equation for the Rasch model so that it now has a discrimination parameter. Plot ICCs with different discrimination parameters to see the impact of manipulating alpha.
#Calculate prob as a function the thetaid =1# item iddelta <-1#item difficulty is 1theta <-seq(-3,3,0.01) #a vector of theta from -3 to 3 in steps of 0.01irt_function <-function(alpha, theta, delta){}
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