[Rasch] Models of unidimensional constructs have their limitations: what is a "good thing"?

Lang, William Steve WSLang at tempest.coedu.usf.edu
Tue Nov 6 23:42:08 EST 2007

I believe this conversation is on track!  Just as "reading comprehension" was bogged down in a complex search for "variables" when Lou Bashaw and Jack Stenner envisioned a usable measure that spurred a new wave of research, Rasch "forces" the scale to understand and map the relevant construct at a targeted level. 

Judy Wilkerson and I also witnessed an endless and unresolvable debate about the complexities of "teacher dispositions" based on morals, ethics, and personality that resulted in useless attempts to measure the construct.  (See "Teacher Dispositions: Building a Teacher Education Framework of Moral Standards", "Teacher Dispositions: Envisioning Their Role in Education", and "Dispostions in Teacher Education" - all recent books.)  Decades of debate could not deconstruct the variables.  The solution was to validate job-related and predictive measures based on a defined and mapped construct that doesn't attempt to sub-divide all the possible variables. (I hope that Lou realizes that I was listening in that class!)

Now that the disposition scales are becoming available, appropriate research and discussion make sense.  Without the measures, you have an endless "factor analysis nightmare" based on any individual's idea du jour.  Until a useful measure at some useful dimensional level starts the process, it is unlikely that a synthesis of "stuff" will magically become useful.  I think this appears to happen in personality, theories of creativity, theories of intelligence, and likely some of the health care measures. (Does anyone remember the Functional Independence Measure discussion on thresholds in rmt?)

When the "model", such as 3PL, allows the measures to "play around" with the parameters so that there is a false fit, you are off on the invalidity slippery slope.  Rasch seems (to me) to force the measures to explain the construct AT THE LEVEL OF INTEREST.  Once it is working, there are plenty of opportunities (FACETS, etc.) to break down and aggregate the real constituent parts!

More importantly to those of us who are looking at "complex human variables", it is the DPF analysis that seems to inform the construct interpretation in ways that simply are not envisioned in the original scale development!  If you look at "Item Response Theory" (Baker & Kim) or "Item Response Theory for Psychologists" (Embretson & Reise), you hardly see any mention of keyforms, kidmaps, or construct maps that are familiar to us Rasch folks.  I don't think you can EVER VALIDATE (or interpret) the construct or sub-constructs of these complex variables without the person profiles which are completely confounded in 2PL and 3PL models!

Judy and I will repeat, "Rasch puts the people in assessment".  If you can't develop the valid unidimensional constructs at some useful level (especially complex human ones), then you'll likely never have the research tools that encourage one to break out of the tail-chasing debates and identify the "real variables" that actually exist in the data.

This is entirely applied and practical, regardless of the mathematical logic of sufficiency, specific objectivity, and independence even though I'm glad that others more talented than me have provided that logic.

Steve Lang
University of South Florida St. Petersburg

-----Original Message-----
From: rasch-bounces at acer.edu.au on behalf of Rod O'Connor
Sent: Tue 11/6/2007 12:25 AM
To: Trevor Bond; rasch at acer.edu.au
Subject: Re: [Rasch] Models of unidimensional constructs have their limitations
Hello Trevor, very nice to hear from you (I recall your earlier 

I have no doubt one could fit global HQoL data to a single Rasch scale 
(any notion can be/is one construct, and multiple items can generally be 
identified), however I fear that people might think this is 
automatically a good thing and that having done so the healthcare 
researcher has 'done their job'.  Unfortunately in principle such a 
scale might not be particularly helpful in 'improving healthcare', 
potentially offering little more than a 'do you feel better' question 
(except the scale would have more than one item).  My view is that a 
HQoL measure useful for healthcare improvement needs to not only provide 
an indication of extent but also play a diagnostic role (allow 
'formative' as well as 'summative' evaluation), identifying where and 
how a change has occurred, and hence where treatment could usefully be 

Of course it all depends on the criteria by which one would assess 
success for the measure, which should always be set-up first.  
Personally I think an overtly multi-dimensional assessment is likely to 
be needed, with an accompanying complex scoring/combination rule to 
predict values consistent with patient judgement.

Warm regards,

_Trevor Bond said the following on 6/11/2007 3:34 PM:_
> mea culpa...denied not denied (sorry Dr Freud...it was a slip)
>> Dear Rod
>> HRQoL is one of the areas where Rasch is having a major impact, if I 
>> can judge by the articles I am asked to review.
>> Are multidimensional states a composite of unidimensional states?
>> Why would we ever expect that any human condition could be DEFINED by 
>> one dimension?
>> thanks for your prompt!
>> TGB
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