1. Can you do an IRT/IFA model with only 1, 2 or 3 Likert scale items. What is the minimum number of items required? Is it like CFA in which you use the concepts of just identified/over identified to determine if you have enough items?
2. What is the formula used for outputted thetas using SAVE = FSCORES?
1. The rules are the same as for CFA. You need three factor indicators for a just-identified model. With two factor indicators, the model is not identified unless it borrows information from another factor.