how to fix item slope and item difficulty in mirt.model()?

[Emily]

This is my model: 

 mirt.model('F1 = 1-300

                        F2 = 1-600')

F1 is math ability and I want to fix the item slopes to be 1 and fix the values of item difficulty parameters.

How can I specify them in mirt.model()? 

[Phil Chalmers]

Hi Emily,

You'd have to use the FIXED and START arguments in the mirt syntax. So something like

 mirt.model('F1 = 1-300

                    F2 = 1-600
                    START = (1-300, a1, 1.0) 

                    FIXED = (1-300, a1)')

Regarding the difficulty parameters, I'm not sure how you want to fix those, but there are other ways if the syntax proves to be too cumbersome (e.g.., pass the pars = 'values' argument to mirt() to obtain a data.frame of starting values)

Phil

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