Item location in IRT PreviousNext
Mplus Discussion > Categorical Data Modeling >
 Tony LI posted on Thursday, June 11, 2009 - 7:03 am
I am wondering is there a good way to infer item location using IRT parameters.

I am fitting 2PL GRM (Mplus)to a 12 items scale and intend to use thresholds to derive the items' location on the latent trait. The scale has both positive and negative items (I have not reversed the response score for negative items when estimating).

The response options are 4 point Likert scale ranging from "Not like me at all" to "Exactly like me". I use the last threshold as approximation of item location, which seem to work ok for positive items.

However, I seem to have problem with the negative items. For example a negative item has thresholds of -0.066, 1.446, 2.296. Using the 3rd threshold 2.296 as item location does not make sense, as a negative item should be on the negative end of the trait continnum. Intuitively, I am thinking should I reverse the threshold to -2.296 or should I use
-0.066 to approximate the item location. Or am i completely on the wrong track?

 Bengt O. Muthen posted on Thursday, June 11, 2009 - 11:02 am
For the negative items I assume you get negative loadings. So a threshold of approx. 0 (-0.066), implying that most people are in the Not like me at all category, refers to the category Exactly like me if you had reversed the item before analysis and gotten a positive loading. Which is a complex way of saying that I think you should use -0.066 to be in line with what you do for the positive items.
 Jeongwook CHOI posted on Friday, August 02, 2019 - 2:34 am

I want to know which is b parameter under GPCM.

I analysed Generalized Partial Credit Model using Mplus8.

Mplus8 showed two paramter. the one is steps in MODEL RESULT and the other is Item Location in IRT PARAMETERIZATION.

I know that Item location is the mean of item step parameter.

But it's different between Item Location in IRT PARAMETERIZATION and mean of steps parameter that I calculated.

What is the Item Location?

and I'll interpret steps to item category parameter?

or I'll calculate item category parameter like this: Item Locatin - stpes?

please reply

thank you
 Tihomir Asparouhov posted on Friday, August 02, 2019 - 3:13 pm
See equation (27) in
The steps are the tau parameters and Beta is the regression parameter reported in Mplus as U on X or Eta by U. The Mplus output corresponds to that parameterization. Take a look also at User's Guide example 5.5.
 Jeongwook CHOI posted on Friday, August 02, 2019 - 10:20 pm
Thanks for your reply.

I have a question.

I didn't look Beta in result.

Where Beta is in MODEL RESULT or result of TECH4?

Thank you
 Tihomir Asparouhov posted on Monday, August 05, 2019 - 9:32 am
It should be in model results. Look for U on X.
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