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From: jamesw@uhunix.uhcc.Hawaii.Edu (James Williams)
Subject: Re: Membership functions and non-existant data.
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Date: Thu, 27 Apr 1995 09:50:43 GMT
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In article <3nnbd8$32t@nntp.crl.com> Richard Berman <sts@crl.com> writes:

>An "unmeasured variable" is something I am a bit "fuzzy" about -- exactly
>what do you mean by this?
>
>------------------------------------------------------------
>Richard Berman                       Vox: (213) 258-7525
>------------------------------------------------------------
>

Unmeasured variable is exactly what it sounds like.  Let's say you
were building an expert system to diagnose a disease.  There are 
a number of clinical tests used for diagnosis, but no doctor orders
all the tests.  

So a membership function of the tests not done should indicate that 
the test was not done.  It should not say that the test result was
0 or any value up to 1.  

Example: Cholesterol, trigliceride, and glucose blood levels are 
	ordered.  But the lab tech drops the vile for the Cholesterol
	assay.  So the test result comes back:
		Cholesterol: NA
		Triglyceride: 123
		Glucose:  120

	If the membership function high(cholesterol) defaults to 0 then
	it is saying that the cholesterol level is not high low when the
	patient could have had an astronomical level.  

I think there is a real semantics problem in fuzzy logic which revolves
around the concept of truth.  Lets say I walk down to the harbor take
a look at the water and phone you to tell you the tide is low.  Then
you would asign membership functions high(tide)=0 and low(tide)=1.
But you get another phone call from death valey (where tide calendars
are not sold) and my twin brother tells you the tide is high.  Then
what do you assign to your membership functions high(tide) and low(tide)?

My brother has no idea what the tide is.  So I say in this case:

	high(tide) = 1 with 0 certainty.
	low(tide)  = 0 with 0 certainty.

Hope this is less fuzzy (to understand).

------------------------------------------------
Karl E. Ihrig
Biggart House
Broadway
Belfast BT12 6HG
Northern Ireland


