[Rasch] Models of unidimensional constructs have their limitations

Rod O'Connor rodocon at rodoconnorassoc.com
Tue Nov 6 16:25:39 EST 2007

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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