[Rasch] Question about poor reliability

Mike Linacre mike at winsteps.com
Thu Dec 26 09:46:25 AEDT 2019

Hi Gustaf:

Thank you for your question.

Three things influence the reliability and separation or your students 
(for Rasch or CTT):
1) the spread of the student measures/scores (not the number of 
students): more spread, higher reliability
2) the number of ratings of each student: more ratings, higher reliability
3) the number of categories in the rating scale: more categories, higher 

Gustaf, based on your description, there is good spread of the spellers' 
measures/scores, and there are plenty of categories in the rating scale. 
It looks like there are only 2 ratings of each speller. An average of 4 
ratings of each speller are probably needed.

Mike L.

On 12/26/2019 3:29 AM, Gustaf Bernhard Uno Skar wrote:
> Dear Rasch Community,
> I turn to you with a reliability question. I have conducted a 
> MFRM-analysis (in the FACETS software), using the rating scale model. 
> I had three facets: students, raters, and dimensions of text quality 
> (there were eight dimensions). Each dimension was rated by at least 
> two raters on five-point scales. The separation reliability for 
> students was good: .94.
> In a subsequent analysis, I pulled one dimension out to create two 
> measures. The first (call it a limited writing proficiency measure) 
> still showed high enough separation reliability (.94), but the second 
> (spelling) indicated too low reliability (0.00 according to the 
> Software output). Now here’s the question: what should I do with the 
> spelling measure? To be sure, there is a lot of variation in the data 
> (some kids are excellent spellers, some not so much).
> Using the raw scores is one option, using the logit values another. 
> Using raw scores will have some unwanted consequences (such as having 
> a measure limited to the steps 1, 1.5, 2, 2.5 etc). Using logits, 
> however, may not seem justifiable because the analysis was not able to 
> reliably separate the students.
> Any food for thoughts or hints about relevant papers/readings will be 
> much appreciated.
> Merry Christmas
> Gustaf
> ---
> Gustaf B. Skar, Ph.D.
> Norwegian University of Science and Technology (NTNU)
> ________________________________________
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> email: Rasch at acer.edu.au
> web: https://mailinglist.acer.edu.au/mailman/options/rasch/mike%40winsteps.com

Mike Linacre, mike at winsteps.com or winsteps1234 at gmail.com
Winsteps 4.4.8 and Facets 3.83.0 - www.winsteps.com 
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