[Rasch] One partial-credit item or multiple dichotomous items?

David Andrich david.andrich at uwa.edu.au
Tue Mar 3 12:13:26 EST 2015

Stuart. I think in the last question, you answered how to do it. There is a dependence and it is a logical, deterministic dependence, hence it should be scored as partial credit, not two independent responses. I am reminded, and the analogy is useful I think of an item in health outcomes of quality of life: item A - Can you walk X metres, Item B can you walk X+a metres. Clearly if you can walk X+a you can walk X - the dependence is logical and deterministic. Therefore, this is a single item with partial credit scoring, not two independent items.
Hope this helps

David Andrich, BSc MEd W.Aust., PhD Chic, FASSA 
Chapple Professor david.andrich at uwa.edu.au
  Graduate School of Education
The University of Western Australia
M428, 35 Stirling Highway, 
Crawley, Western Australia, 6009
Telephone: +61 8 6488 1085;   Fax: +61 8 6488 1052

-----Original Message-----
From: rasch-bounces at acer.edu.au [mailto:rasch-bounces at acer.edu.au] On Behalf Of Stuart Luppescu
Sent: Tuesday, 3 March 2015 5:42 AM
To: rasch at acer.edu.au
Subject: [Rasch] One partial-credit item or multiple dichotomous items?

Hello Fellow Raschies, I'm working on a test with some partial-credit items. The coding is something like 0: Did not understand at all; 1: 
Understood incompletely; 2: Understood completely.
I'm debating with the test developers about whether to make these individual items scored {0, 1, 2} or two dichotomous items: item 1: 
understood at least partially; item 2: understood completely. Is there an advantage to using one or the other approach?
Also, if we go with the two-dichotomous-item approach, how should the second item be scored if the student did not understand the question at all? That is, does it make a difference if the two items are scored 0, 0; or 0, omitted?

Stuart Luppescu
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