It's How the Data will be Used that Counts

Kevin Thiele kevin.thiele at BIGPOND.COM
Tue Dec 4 17:49:25 CET 2001

----- Original Message -----
From: "Steve Shattuck" <Steve.Shattuck at CSIRO.AU>
Sent: Tuesday, December 04, 2001 11:31 AM
Subject: It's How the Data will be Used that Counts

| A couple of points:
| Kevin suggests a rule "<states>s cannot have <characters>s as siblings".
| But this is what DELTA calls Dependencies, it represents the state that
| controls a character.  This would seem to be a good thing (and may be very
| important).

No, this is not equivalent to dependencies, although at first sight it seems
similar. It's a tree-topology rule: the rule "<states>s can have
<characters>s as siblings" allows certain topologies for the
data-representation trees that would be preluded by the rule "<states>s
cannot have <characters>s as siblings". As I said, I don't know whether the
rule's a good or necessary one or not.

Dependencies establish relationship rules between different parts of the
tree. Sometimes these rules affect parent-sibling relationships as in the
example, but sometimes not - that is, dependencies are more complex than can
be handled by a topology rule. In dependencies, there may be several
controlling states for a character, and these may or may not occur on nodes
antecedent to the node in question.

An example (from a key of mine):

the character:

Trap structures (on carnivorous plants)
 submerged or underground bladder-traps
 sticky hairs
 irritable leaf-blade segments

has controlling states in several other characters, viz:

Nutritional strategy
 neither carnivorous nor parasitic*

 terrestrial herb
 aquatic herb

In the key, if a user chooses a state marked * the character "Trap
Structures" will disappear because of the dependencies. Note that the
dependency on tree and shrub is not a logical one but a contextual one - it
just happens that no trees or shrubs are carnivorous, so the dependency's
useful but not logically necessary.

I don't think these dependency relationships can be expressed merely by the
topology of the tree, as you suggest. We will need to deal with dependency
rules as a separate issue.

Cheers - k

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