Min # items irt/factor score formula PreviousNext
Mplus Discussion > Categorical Data Modeling >
 ljc posted on Tuesday, May 14, 2013 - 1:17 pm
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?

Thank you.
 Linda K. Muthen posted on Tuesday, May 14, 2013 - 1:59 pm
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.

2. See Technical Appendix 11.
 ljc posted on Wednesday, May 15, 2013 - 7:48 am
I thought it might be different since 1 Likert item with 5 levels, actually goes into the model as 4 dichotomous items and error variances aren't estimated. I'll stick with a minimum of 3. Thank you.
 Linda K. Muthen posted on Wednesday, May 15, 2013 - 8:48 am
I am assuming each Likert item is one variable. This is how I would treat them.
 ljc posted on Wednesday, May 15, 2013 - 10:23 am
I thought that was what the CATEGORICAL ARE command did, made the one 5 level Likert item go in as 4 items, so you get 4 intercepts/thresholds and 1 loading for each Likert item.

While excluding the CATEGORICAL ARE command would make it CFA so the Likert item goes in as 1 item giving you 1 intercept and 1 loading.
 Linda K. Muthen posted on Wednesday, May 15, 2013 - 11:05 am
It does not create four dummy variables. Each Likert item gets one factor loading and the number of categories minus one thresholds.
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