[Rasch] Number of categories

Stone, Gregory gstone at UTNet.UToledo.Edu
Sat Jan 2 05:46:41 EST 2010

The number of rating scale points should be determined both by the theory and rationale of increase from less to more, and the ability of the instrument maker to define points consistently distinguishable by respondents.  In models that do not evaluate the use of rating scale points by respondents, simply adding more may lead to the faulty observation of improved precision.  However, in reality, more does not equate to greater precision.  The better questions to ask would be not whether moving from 5 to 8 would improve the measure, but rather:

Are the respondents using the current 5 point scale appropriately?

Do each of the current 5 points have clear and unique descriptors that are easy for respondents to use?

If additional points were to be added, could they be defined and described to respondents in a way such that respondents would consistently make use of each point across the scale?

This is likely the more critical question, assuming the first two are true.  We can add 3 points or 30 to a scale.  The problem is that human respondents simply cannot retain and consistently apply definitions associated with the points along lengthy, poorly defined scales.  Only when each point on the scale is unambiguous does more = potentially better. 

As to the map, if there are gaps, then there are gaps in how your items cover the construct and more points will likely not solve that problem in any way.  

Gregory E. Stone, Ph.D., M.A.

Associate Professor of Research and Measurement
Judith Herb College of Education   University of Toledo, MS #921
Toledo, OH 43606   419-530-7224

Board of Directors, American Board for Certification of Teacher Excellence     www.abcte.org
For information about the Research and Measurement Programs at The University of Toledo and careers in psychometrics, statistics and evaluation, email gregory.stone at utoledo.edu.

-----Original Message-----
From: rasch-bounces at acer.edu.au on behalf of Purya Baghaei
Sent: Thu 12/31/2009 8:48 PM
To: rasch
Subject: [Rasch] Number of categories
Dear colleagues,

In a Likert scale analysis with 5 points using Rasch RSM, the map

showed that most of the items are clustered at the lower end of the

scale and there are only two items at the top with many respondents

falling at the upper part of the scale where there are no items. Well,

the solution is to add some items to cover the gap

The other finding was that the distance between the last two

thresholds was more than 4 logits. Although this is within the

acceptable distance of 1.4 to 5 logits (Linacre, 1999) it's still a

wide gap.

I was just wondering if increasing the number of categories from 5 to

say, 8 can solve the problem of lack of items at the upper part of the

scale. Because it seems that respondents are capable of distinguishing

more subtle levels of the construct at the upper end of the scale. Can

increasing the number of categories be a substitute for writing more





Purya Baghaei, Ph.D

English Department,

Islamic Azad University,

Ostad Yusofi Str.

Mashad, Iran.

Phone: +98 511 6634763


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