[Rasch] assessing construct validity over time

Trevor Bond trevor.bond at jcu.edu.au
Wed Jul 25 10:18:14 EST 2007

>Dear Alexander,
>I have a question I hope I can get a helpful answer to.
>I'm investigating retest effects for an intelligence test 
>(dichotomous scoring) over 4 test administrations. I would also like 
>to check if the construct measured with my items changes over time. 
>At every time point, I have sets of 15 items, and I'm using the same 
>15 items at points 1 and 3, and a set of item isomorphs at points 2 
>and 4. So essentially, every subject delivers 60 answers, and 2 
>answers each for 30 items, respectively. My question is how can I 
>use Rasch methodology to assess the construct validity of these 
>items over the 4 time points? Is it legitimate to simply try to fit 
>one Rasch model to all 60 items? Or what can be done in this case? I 
>would be very thankful for any advice.

What is the nature of the construct you are validating?
 From whence do the items come?
What do they instantiate?
Why are they written / arranged like that?
Is there an expected/hierarchical arrangement of difficulty?

Do all 30 items fit the model when administered at once?
Checking the invariance of the estimates can be done a number of ways:
Chapter B&F 2nd ed shows how and provides an XL spreadsheet on the CD

15 items will say precious little about your kids (large SEs)
What willl your sample size be?
What will be the distribution of the latent trait in that sample/

Perhaps you should look at Sam Messick on construct validity...

Hope that helps.
Trevor G BOND Ph D
Professor and Head of Dept
Educational Psychology, Counselling & Learning Needs
D2-2F-01A EPCL Dept.
Hong Kong Institute of Education
10 Lo Ping Rd, Tai Po
New Territories HONG KONG

Voice: (852) 2948 8473
Fax:  (852) 2948 7983

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