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Re: Using boolean value vs class

From: Antoine Zimmermann <antoine.zimmermann@emse.fr>
Date: Wed, 25 Sep 2019 12:10:39 +0200
To: Michael F Uschold <uschold@gmail.com>, Hugh Glaser <hugh@glasers.org>
Cc: "semantic-web@w3.org" <semantic-web@w3.org>
Message-ID: <54f78522-c091-c36e-57f4-e15be46150ed@emse.fr>
I'm a bit confused by what you say, but I have the impression that you 
are saying that "isDeprecated" is, after all, similar to a property like 
"hasTag". Said differently, it seems you are saying that if we dump the 
property "isDeprecated" in favour of a named class, then by the same 
arguments we should dump the property "hasTag" in favour of a class. 
Then, considering that a property "hasTag" can be useful and, frankly, a 
good modelling option, you conclude, methinks, that the arguments for 
dumping "isDeprecated" in favour of a class are not so clear-cut.

If this is your reasoning, then I disagree. I hope you realise that the 
situation with a boolean property is really different than the situation 
with a property like hasTag. First, "tags" do not form a two-valued set. 
You cannot say the tag is either "winter" or "summer", and nothing else. 
Second, there is no expectation that something must have a tag and there 
is no expectation that something has exactly one tag.

Even if you have a two-valued set, say D = {"A", "B"}, and a property 
:prop that has range D, then there is nothing special about:

  ex:this  :prop  "A", "B" .

However, with all boolean properties that I've seen, there is clearly an 
expectation that:

  ex:this :booleanProp  true, false .

is an error. There is also the expectation that if a value is not 
present for the property, then it is certainly false. These expectations 
do not come for free in RDF/OWL.

Even if you add the closed world assumption, you cannot infer that a 
missing value for :booleanProp means "false"^^xsd:boolean. In fact, the 
closed world assumption would lead you to say that *neither*:

  ex:this  :booleanProp  true .


  ex:this  :booleanProp  false .

are true. With a class :Deprecated and a closed world assumption, things 
that are not typed with :Deprecated would be inferred to be not deprecated.


Le 25/09/2019 à 07:11, Michael F Uschold a écrit :
> This is an important discussion, as this modeling question arises all 
> the time.
> I agree that Boolean data properties are not a great option. This is 
> explained in this blog: Why Not to Use Boolean Datatypes in Taxonomies 
> <https://www.semanticarts.com/why-not-to-use-boolean-datatypes-in-taxonomies/> 
> by Dave McComb.
> OWL inference may be a red herring here.You may not be running OWL 
> inference over a large ABox of documents?More likely, you are just going 
> to run inference on the TBox and then load triples into a triple store 
> and use whatever reasoning is provided by that vendor (highly variable, 
> and certainly not OWL2-DL).
> Creating a class called Deprecated will work, but may not be the best 
> solution. First, it goes against common practice for naming a class. 
> Common names for classes include “Person” and “Document”. An instance of 
> the first class is a person. An instance of the second class is a 
> document. However an instance of your proposed class is not a 
> ‘deprecated’. Rather it is a deprecated thing. If you named the class 
> DeprecatedThing, the naming convention would be respected. However, 
> that’s not a very satisfying class.The reason points to more fundamental 
> issue, aside from naming.
> The main purpose of an OWL class is to represent a set of things that 
> are all the same kind (person, document).Nobody thinks of being 
> deprecated as signifying a different kind of thing. It’s more analogous 
> to a tag for a photo.If you tag a photo with “winter”, this gives rise 
> to a set of things tagged with “winter”. One could represents that set 
> as an OWL class, just like one could represent the class of all 
> deprecated things with the class DeprecatedThing.But these sets do not 
> represent a kind of thing one would want to represent as an OWL class. 
> Rather, being deprecated or not is a characteristic or facet of a thing. 
> Documents and products and lot of things can have many facets.
> There is a third alternative that we use in our enterprise ontologies.I 
> would create a class called say DeprecationIndicator with two instances: 
> isDeprecated and isNotDeprecated. These are really categories: something 
> is deprecated or not.There are typically many such facets and each has a 
> set of values. There might be a facet called Color for cars or 
> iPhones.An individual car or phone would have a color and there could be 
> several instances of the class Color (rose, midnight green, etc). An 
> advantage of this approach is that you avoid unnecessary proliferation 
> of properties, one for each facet. You do not need two properties one 
> for hasColor and one for hasDeprecationIndictor. Rather you can just use 
> a single property, say isCategorizedBy.This is further explained in this 
> blog: Buckets, Buckets Everywhere, Who Knows What to Think? 
> <https://www.semanticarts.com/gist-buckets-buckets-everywhere-who-knows-what-to-think/> 
> by yours truly.
> On Tue, Sep 24, 2019 at 7:03 AM Hugh Glaser <hugh@glasers.org 
> <mailto:hugh@glasers.org>> wrote:
>     Very interesting question, thanks - it helps me explore my
>     understanding.
>     Sorry - as I have said, I'm not really very good on this stuff, but
>     I do like to try to understand.
>     Antoine, some of what you say puzzles me.
>     Looking at class :Deprecated
>      > The second model with a class :Deprecated ensures that an entity
>     is either of type :Deprecated, or not.
>     Is it not more properly the case that an Entity is either of type
>     :Deprecated or we don't know? (Open world)
>     So the boolean version seems to perhaps give me a richer way of
>     recording knowledge.
>     To model the boolean equivalent, you could also have a
>     :notDeprecated class.
>     And then you would have the same four categories for the class
>     version as you have for the boolean version.
>     (Not saying this is good!)
>     [Hang on - I have just realised that Mikael makes no suggestion that
>     he will ever assert "false" - so your introducing the "false"
>     categories (3 & 4) is like me introducing the :notDeprecated class.]
>     Although I worry about your argument here, I think that the general
>     principle may well be very good.
>     If you see booleans, especially where they always seem to be "true",
>     it is a flag that maybe a class should be used.
>     (This is very similar to seeing "= true" in an expression in a
>     programming language, someone isn't thinking right :-) )
>     I usually view an rdf:type triple as nothing special compared with
>     any other.
>     You assert them and match them just the same.
>     It just so happens that "we" have chosen that we can do
>     sub-classing, and so if we do that, we get some special magic that
>     can happen, which doesn't happen with everything else.
>     And that is sometimes very useful, although it can make things quite
>     hard to get the hang of.
>     Then, as you say, there are a whole bunch of practical questions
>     about efficiency of stores and reasoners when you do things in
>     different ways.
>     But, as with programming, most efficiency things should be left to
>     the system implementation, and the source should be modelled in the
>     most understandable and maintainable way.
>     Best
>     Hugh
>      > On 24 Sep 2019, at 13:48, Antoine Zimmermann
>     <antoine.zimmermann@emse.fr <mailto:antoine.zimmermann@emse.fr>> wrote:
>      >
>      > Mikael,
>      >
>      >
>      > These two options definitely affects reasoning.
>      >
>      > If you have a property :isDeprecated, then any entity can fall
>     into 4 disjoint categories:
>      >
>      > 1. The entities that have no value for :isDeprecated.
>      > 2. The entities that have value "true" only.
>      > 3. The entities that have value "false" only.
>      > 4. The entities that have both values "true" and "false".
>      >
>      > Moreover, if the range of the property is unrestricted, it can
>     have all sorts of literal values, in any combination.
>      >
>      > If you want to make sure that all entities have exactly one of
>     "true" or "false" as value for :isDeprecated, you need to introduce
>     a cardinality axiom, which increases the complexity of reasoning
>     (and you need to find a reasoner that supports cardinality
>     restrictions on datatype properties).
>      >
>      > The second model with a class :Deprecated ensures that an entity
>     is either of type :Deprecated, or not. This comes for free with any
>     reasoner that supports a logic as simple as RDFS, without extra
>     axioms. Many more reasoners support axioms made on classes than
>     axioms made on literals and datatype properties. It's easier to
>     define subclasses of deprecated documents, for instance.
>      >
>      > In general, when I review an ontology document, I mark all use of
>     boolean properties as a mistake. Usually, boolean properties comes
>     from adopting a programming approach to ontology engineering rather
>     than a knowledge representation approach (that is, it uses the
>     ontology as a data structure for computation rather than as an
>     information model about the world, for knowledge interchange).
>      >
>      > However, when you have to go back and forth between an existing
>     data model such as tabular data etc. and RDF, it can be convenient
>     to translate booleans to booleans, so there can be exceptions to my
>     rule of thumb of excluding all boolean properties.
>      >
>      >
>      > Best,
>      > --AZ
>      >
>      > Le 24/09/2019 à 13:57, Mikael Pesonen a écrit :
>      >> Hi,
>      >> lets say we have documents and we want to say wheather they are
>     valid or deprecated. There are two ways to do this:
>      >> :doc1 a foaf:Document ;
>      >>     :isDeprecated "true"^^xsd:boolean .
>      >> or
>      >> :doc1 a foaf:Document ;
>      >>     a :Deprecated .
>      >> Are there some different implications on the use? Does is affect
>     OWL reasoning, for example?
>      >> Mikael
>      >
>      > --
>      > Antoine Zimmermann
>      > Institut Henri Fayol
>      > École des Mines de Saint-Étienne
>      > 158 cours Fauriel
>      > CS 62362
>      > 42023 Saint-Étienne Cedex 2
>      > France
>      > Tél:+33(0)4 77 42 66 03
>      > Fax:+33(0)4 77 42 66 66
>      > http://www.emse.fr/~zimmermann/
>      > Member of team Connected Intelligence, Laboratoire Hubert Curien
>      >
>     -- 
>     Hugh
>     023 8061 5652
> -- 
> Michael Uschold
> Senior Ontology Consultant, Semantic Arts
> http://www.semanticarts.com <http://www.semanticarts.com/>
>     LinkedIn: www.linkedin.com/in/michaeluschold 
> <http://www.linkedin.com/in/michaeluschold>
>     Skype, Twitter: UscholdM

Antoine Zimmermann
Institut Henri Fayol
École des Mines de Saint-Étienne
158 cours Fauriel
CS 62362
42023 Saint-Étienne Cedex 2
Tél:+33(0)4 77 42 66 03
Fax:+33(0)4 77 42 66 66
Member of team Connected Intelligence, Laboratoire Hubert Curien
Received on Wednesday, 25 September 2019 10:11:05 UTC

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