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[protege-owl] Query: Subclasses or Sibling Disjoint classes?

william fitzgerald wfitzgerald at tssg.org
Mon Apr 2 02:32:52 PDT 2007


Dear Timothy and OWL users,

1) I agree with your first point entirely and the whole point of using 
OWL-DL is to have poly-hierarchies sorted automatically via the reasoner.

2) You are spotting some issues with the NamedPizza layout also. I just 
fired up protege and made all these NamedPizza's complete classes. there 
is no major difference in the outcome ( alot of classes now defined 
under UnClosedPizza of course) and in fact the hierarchy looks more 
complete.

3) yep you are correct. I was only referring to the idea of having a 
AmericanHot defined as a subclass of American but i didn't bother 
thinking about the restrictions. Placing AmericanHot under American will 
cause an error due to the fact that the restrictions (hasTopping) on the 
now parent class American will kick in under the reasoner.

So what we conclude?

Question 1: it seems that all these NamedPizza's are in themselves 
closed for finalized (disjoint siblings and specific restrictions) to 
what they are made up of, so why not make them complete classes?

It seems that an AmericanHot pizza is infact quite different in the 
ontology model to American pizza despite the fact they share a lot of 
the same principles! it seems natural and more likely in a programming 
environment to have a class American and a more specialised class called 
AmericanHot which inherits its parents qualities plus its own and 
possibly overrides some of the parent restrictions.

Question 2: hence in OWL-DL, can a sublcass override parent classes 
restrictions in Protege etc.?

it seems the answer is no, and hence why the Pizza developers make both 
American and AmericanHot disjoint sibling classes. However given 
question 2 is probably a NO, then why was Question 1 not applied?

maybe alot of the design issues are down to OWL-DL limitations and a 
complete mapping to OO programing does not overlap in a precise way 
(code optimzation).

hence in java you would have a 2 separate java classes American and 
AmericanHot driven by the Pizza Ontology rather than having 2 classes 
American and AmericanHot with AmericanHot as a subclass of American and 
having AmericanHot override properties of its parent class if required.

that is:
class American{ //class desription}
class AmericanHot {//class description}

rather than:
class American{ //class desription}
class AmericanHot extends American {//class description}

I welcome any comments in relation to this or any other features that 
may come to mind?

All I am trying to do is get into the mind set of correct modeling using 
DL and OWL-DL, and in a correct and formal way. I would like to be able 
to stand over any model I develop and have exact answers to why its 
modeled in a particular way.

regards,
Will.



Timothy Redmond wrote:
>
> I see three issues.  The first thing is that in OWL we shouldn't need 
> to build all possible subclass relationships when we write the 
> ontology because the inference engine will calculate them for us.  I 
> am not a modeling expert but this seems like this is a good principle 
> for OWL modeling.  This doesn't help with primitive concepts but for 
> defined concepts this may be useful.
>
> Second, I have a little trouble with the NamedPizza hierarchy.  It is 
> interesting that all of these guys are primitive - none of them are 
> defined.  This means that there is something about a pizza that makes 
> it an american pizza that is not defined in the ontology.  Thus it is 
> unclear whether american pizza's and american hot pizza's should be 
> disjoint because it is not clear to me what is being captured in this 
> distinction.
>
> Finally, and you might view this as nitpicking, even if you ignore the 
> disjointness axioms attached to the american pizza and the american 
> hot pizza, it is impossible for these two types of pizzas to 
> intersect.   The american pizza has a restriction axiom that states 
> that it can only have mozzarella, peperoni sausage and tomato 
> toppings.  But the american hot pizza has a restriction axiom that 
> states that it must have a JalapenoPepper topping.
>
> -Timothy
>
>
>
> On Mar 30, 2007, at 6:19 AM, william fitzgerald wrote:
>
>> Dear OWL Gurus,
>>
>> In developing my own OWL-DL ontology similar in structure to the PIZZA
>> OWL-DL ontology, I find myself faced with a question as to why certain
>> classes have not been defined as subclasses.
>>
>> For example in the NamedPizza class we have 2 subclass siblings called
>> American and AmericanHot.
>>
>> Clearly the AmericanHot is a more specialized class to that of the
>> American. So why has it not been classed as a subset of AmericanPizza?
>>
>> One argument is that a hot American pizza is not an American pizza
>> (because one will burn the mouth off you and the other will not) hence
>> the reason for not subclassing!
>>
>> However if we take another example we can see that under Vegetable
>> toppings we have defined HotGreenPepperTopping as been a subset of
>> GreenPepperTopping. In this case we do not create sibling classes but
>> rather we subclass.
>>
>> So do you see my predicament!!!
>>
>> Is there a hint of a flaw in the ontology (remembering of course that
>> the pizza ontology is only for demonstration purposes) or is there valid
>> reasons for subclassing in one area and making disjoint siblings in 
>> another?
>>
>> I realize that the structure of the pizza ontology is very intuitive and
>> very close to how we naturally visualize the make up of Pizza's but
>> taking these principles to other domains can often be trickier.
>>
>> regards,
>> Will.
>>
>> --William M. Fitzgerald,
>> PhD Student,
>> Telecommunications Software & Systems Group,
>> Waterford Institute of Technology,
>> Cork Rd.
>> Waterford.
>> Office Ph: +353 51 302937
>> Mobile Ph: +353 87 9527083
>>
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>

-- 
William M. Fitzgerald,
PhD Student,
Telecommunications Software & Systems Group,
Waterford Institute of Technology,
Cork Rd.
Waterford.
Office Ph: +353 51 302937
Mobile Ph: +353 87 9527083 




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