[Rasch] analysing CTT construct using IRT

Man Xu man.xu at education.ox.ac.uk
Tue Oct 27 01:41:57 EST 2009

Dear Listers,

I am very new to the IRT field so please forgive me if my question is too simplistic.
I am using the PISA 2006 data for my doctoral dissertation and so far I have been using CFA methods for analysing the construct properties the Likert scale data in PISA. All constructs in my study have high level of reliability and mostly demonstrate good multigroup invariance properties under CFA/SEM methods. Recently, I was introduced to the IRT methods and start to development new perspectives on measurement issues of the data at hand.
In PISA, many of the motivational constructs are measured through Likert scales and were measured through well established constructs developed under the framework of CTT. Using IRT method, I found that the difficulties of items in these constructs are often clustered at middle or high levels of the trait. The Test Characteristic Curves are always peaked at one place rather than spread all over the trait levels. I guess this means that the latent variable is not realiable enough for all the levels. So I was wondering if these constructs are valid enough to use for further analysis or whether I should just stick to my old way of analying this data, i.e. using CFA/SEM and treating the likert data as if they were continuous.

Any opinions would be highly appreciated!
Thank you very much.

Best wishes,


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