Notes from list discussion...
I have come across an interesting example of where we might want to
model an ordinal constraint where the values associated with each term
are not unique.
The example is the Waterlow Score
If you look at the bottom-right corner "Major surgery / Trauma" you
will see that two terms have identical values.
The Archetype Editor and VCOV rule in the TRUNK openEHR Archetype profile prevent non-unique values.
a problem with Ian's score is that it's not an ordinal scale, but a partial order. E.g. for the skin type variable, "healthy" is ordered in relation to the other elements, but "tissue paper"* is not ordered in relation to "dry" or "oedematous". In the openEHR Archtype Profile the VCOV rule disallows DV_ORDINALS with identical values in the C_DV_ORDINAL list, thereby only allowing total orders. This could be changed though!
Don't know how this scoring system is used, but "oedematous" and "dry" can not reasonably be seen as terminological synonyms!
I agree that we seem to be mixing up 2 somewhat different ideas here.
One is the 'true' ordinal, where there is a set of ordered terms to
each of which some sort of ordered but otherwise arbitrary integer
value is assigned. If all we are doing is asserting the correct order
of the terms then it is questionable whether anything other than the
index position in the list is necessary e.g 0..n. However there are
many examples of ordered lists in clinical use which have at least
some sort of score attached to each list member. As Daniel has pointed
out, it is somewhat dubious if these really qualify as ordinals and I
agree with Tom's idea that these are really term->value mappings.
I would vote for taking the simple option of removing the 'uniqueness'
requirement. I doubt very much if this would cause any downstream
problems with decision support, in that if unique constraints are
required, these will be fully defined by the archetyping itself e.g a
GCS or Apgar score.