jan mod posted on Thursday, June 18, 2015 - 3:57 pm
Dear Muthen & Muthen,
I'm doing an IRT model with two waves (mathematics, dichotomous variables) (single group design). I want to do a concurrent calibration with 10 common items and unique items for each wave. I want to put both waves on a common scale. 1) is this syntax correct? 2) How can I make an IRT calibrated scale for each wave math (two scores on common scale)?
I would think you use 2 theta variables, one for each time point. And with its own indicators. So something like
theta1 by math1-math10 math111-math120* (a1-a20); theta2 by math211-math220* (a1-a20); math21-math45; theta1-theta2@1;
where you have equalities a1-a20 for those items that are in common and administered at both time points (hence the prefix 1 and 2). The 2 theta variables will be correlated as the default. You ask for factor scores to get the individual theta values.