Tuesday, February 21, 2006

Is there a difference between a person and a door?

In the paper "HL7 RIM: An Incoherent Standard" the RIM is criticized for failing to draw a clear distinction between entities in reality (patients, diseases, treatment acts, ...) and the records of such entities, for example in paper documents.

Mead Walker reacts to this criticism in an email as follows:
The paper puts a lot of effort into discussing HL7's mixing of what you call its "information model" with its "reference ontology". I agree that these are distinct concepts, and that HL7 tends to switch from one to another without warning. However, I do not believe the paper substantiates why this is a problem. I think that is its major flaw. It is almost as if you think the seriousness of the defect is self evident, but it is not. Perhaps, this is an omission that only a philosopher would make. Let me use an analogy. You can look at me to see if I have a mustache; you can also look at a mirror image of my face or at a photograph. In either case, the fact that I do have a mustache is evident, and the question of whether you know that by looking at me, or by looking at the picture is normally trivial. Why do you think it is important to know if the knowledge that is being transmitted comes from the thing itself, or from its representation? ... Isn't this simply a fundamental philosophic conundrum that really is not practically important?

My response was as follows:

Dear Mead,
You are right that this is, for me, a self-evident defect. You are right, too, that it sometimes does not matter where you get your knowledge of mustaches fr0m (though it might, e.g. for forensic purposes; and I would imagine that if you are building a messaging standard then these are precisely the sorts of purposes you should be bearing in mind). The main issue, though, relates to the question not of how we know but of what sorts of things our knowledge is about.

If I told you that your messaging standard cannot distinguish reliably between, say, a human being and a door, then you would rightly be troubled.

Why are you not similarly troubled if I tell you that your messaging standard cannot distinguish reliably between a human being and a social security number?

Or between an actual real-world case of cardiac arrest and the information content of a cardiac unit resuscitation note?

Or between John's elevated blood pressure (which endured for several hours) and the information recorded during an action of measuring this blood pressure at 4.07 pm?

Surely it is a serious problem when one does not know what basic categories of entity one is dealing with, so that one confuses real-world phenomena (which might make you sick) with information captured during the observation of such phenomena which sits inside computers. This latter distinction is, for me, as self-evident as the distinction between a real-life performance of Tosca and the musical score you might buy in a music shop.

But, you might still say, is the distinction of genuinely practical importance? Well, consider blood pressure:

On the one hand there is your blood pressure itself, the real-world phenomenon which obeys the laws described in a medical textbook (which will tell you about systemic arterial pressure, about systolic and diastolic phases, about fluid dynamics, etc., etc. complicated physics and physiology that will be of practical importance e.g. when designing an instrument that can accurately measure blood pressure or when dealing with a patient who has atrial fibrillation). On the other hand there is a blood pressure observation, another real-world phenomenon, but of an entirely different sort, involving factors such as:
  • the position of the patient at the time of measuring (sitting, lying, etc.),
  • the tilt of the surface on which the person is lying,
  • the variation in measured blood pressure with respiration,
  • the instrument used to measure the blood pressure,
  • the size of the cuff if a sphygmomanometer is used,

and so forth, as well as the units in which measurements are taken. (One detailed representation of these factors is here.)

Note that blood pressure itself (including the sort of elevated blood pressure that can kill you) exists entirely independently of instruments or units of measure or acts of observation. Blood pressure itself exists as an endurant feature of the organism from one hour or day to the next (constantly rising and falling even while preserving its identity). A blood pressure observation, in contrast, is a process, which does not endure through time but rather unfolds through time in successive temporal phases. (See here.) Thus the features of the former are quite different from the features of the latter, as different as, say, a potato and the act of eating a potato.

Consider, now, for further evidence of the practical significance of this distinction, that there are some words (e.g. 'systolic') which appear both in textbook descriptions of blood pressure itself and in records of acts of observation. Sadly, even leading experts in medical terminology are often misled by this fact. For the two sets of occurrences of the same term refer to entities in reality which are of an entirely different sort. Descriptions of the first kind would be appropriate to ontologies of pathology (of the what it is on the side of the patient) such as we might hope to find in SNOMED-CT; descriptions of the second kind would be appropriate to message descriptions such as we might hope to find in HL7.

Friday, February 17, 2006

Is there anything positive to be said about HL7 V3 ?

Barry Smith said...

I think we can probably leave it to the HL7's own marketing arm to document positive features of HL7 V3.

I think also that, in the age of internet services, it is becoming increasingly unclear whether the technology of messaging standards is needed at all. But even leaving this aside, HL7 V2 is a reasonably good messaging standard for its scope (mainly pathology results and imaging), and it is ten times simpler than HL7 V3 and already quite widely implemented. Where, then, is the business case for Version 3?

As to the Electronic Health Record, we must bear in mind that HL7 itself does not in fact have an EHR solution. And I am tempted to say that more or less anything that has been designed in response to EHR requirements would represent an improvement on something which does not exist.