Hi Mark,
the openEHR model of 'ordinals' (right or not) is that each ordinal
value is an association of an integer with a symbol (which could be
words, but also anything else like '++'). The numbers are used to make
the ordering computable - in fact this is the definition of the
'ordinal' concept in openEHR - that there is order without otherwise any
proper scaled notion of magnitude.
This data type (DV_ORDINAL) is used in at least two ways (wrongly or
rightly):
1. to model scales of symbols, such as 'none', '+', '++', '+++' and
so on (maybe pain, reaction, urinalysis analytes), where there are
otherwise no numerics built into the published scale - i.e.
someone had to invent them when creating archetypes or other
openEHR artefacts
2. to model scales whose values are primarily expressed as numerics
and whose 'symbols' are just words
3. to model scales of numbers like Apgar and Barthel, where the
primary expression of 'value' is numeric, and the numbers are
meant to be added up, and the 'symbol' is in fact a term, i.e.
explanatory phrase
doing this has the desired effect in all cases of establishing
computable ordering between adjacent members (since computers don't
understand that '++' is somehow 'less than' '+++'). But it is clear that
there are downsides:
* in the first case above, it is us (i.e. archetype modellers)
adding arbitrary numbers to make the scale of values computable
* in the second and third cases, there is no obvious reason why real
numbers should not be chosen, especially in the last case above,
since the values might be carefully chosen to make the subsequent
additions have a certain effect.
At the very least it seems that we need is the ability to use real
numbers as well as just integers. Maybe there is more to it - I
certainly don't have a good enough feel for the use of these kinds of
scales in medicine, nor their implied semantics. With respect to
literature, I have to admit we did not find anything much on ordinal
scales, although we tried to stick to well-known notions of accuracy,
precision, units etc in the design of the main quantitative data types
in openEHR (also proper mathematical notions which lead to meaningful
operations, e.g. + and - being defined on true 'amounts' but not on
dates). If you know of any literature on this topic of scale design, I
am sure it would be helpful here.
- thomas
On 21/04/2010 09:12, Mark Leaning wrote:
>
> I am hesitant to enter this discussion, but here goes.
>
> An assessment scale which assigns a numeric score as well as a
> symbol/term is not an ordinal scale. The symbols do in fact comprise
> an ordinal scale ("very low" < "low" etc). However the numerics
> provide something else -- usually to be added up with other scores.
> The designers of such scales are bound to validate the composite
> numeric scores (cf SF36).
>
> What you then have is ordinal symbol + real number. If the scale
> designer needs to use decimals, that is their choice.
>
> It might be worth looking at the measurement theory literature.
>
> Cheers
>
> Mark
>
> Mark S Leaning PhD**
>
*
*
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