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[protege-owl] How to make a restriction for "A HasPrerequisite (X and Y and Z)" expression

ozg ozgun_yilmaz at ttmail.com
Wed May 28 16:16:02 PDT 2008


I chose OPTION 2 and made every lecture a class, more precisely a subclass of
Lecture. I really want to make restrictions on the lectures not the students
(Really good point you made). Modelling lectures are more important for my
model. I'm only modeling graduate students, so the prerequisites are for
modeling lecture class model. And if a student took the course than he/she
also passed it.
I'm developing this ontology in OWL-DL.
Yes I realized using hasValue is irrelevant.
Now I use allValuesFrom : For "LectureA has prerequisite LectureB" I make a
restriction on HasPrerequisite:
"allValuesFrom LectureB"
And when I try to add prerequisite LectureB instance for the
"HasPrerequisite property" for the instance of LectureA which has 1
prerequisite(LectureB), only the relevant prerequisite class's
instances(LectureB instances) come to choose from. Restriction works. It
seems okay. Right?

But if a lecture has more than 1 prerequisite I "or" the prerequisites which
is not an exactly right solution for the case. I tried and'ing them and then
adding individuals to test(if i mixed up the and's and or's) but it is
irrelevant because it doesn't restrict the range as I wanted.
For ex: "LectureA HasPrerequisite LectureX and LectureY and LectureZ"
I use "allValuesFrom LectureX or LectureY or LectureZ" restriction for
HasPrerequisite property.
And when I try to add prerequisites to the LectureA instance, only LectureX,
LectureY and LectureZ instances come to choose from. But it isn't exactly
right because it makes an union of LectureX, Y, Z classes, so someone can
choose all of the prerequisites from LectureX instances for example. Can't I
make a restriction for "1 LectureX and 1 LectureY and 1 LectureZ" with using
only lecture classes?

By the way I have "takesCourse" property.
What I don't really understand is If a lecture has 1 prerequisite I can
model it right. But if it has more than 1 I can't. Why restrictions using
only lecture classes isn't sufficient? 


Thanks for the advice Thomas Russ, I read and understood your points, you
are saying I should extend this ontology and should include students in my
restricitons.



Thomas Russ wrote:
> 
> 
> On May 27, 2008, at 10:50 AM, ozg wrote:
> 
>>
>> I'm new and developing an university education ontology.
>> I added every lecture as a class to the ontology. And I defined a  
>> property
>> "HasPrerequisite" meaning some lectures can have 1 or more  
>> prerequisite(s).
>> If someone wants to take a lecture he/she must have passed from all  
>> of the
>> prerequisite lectures. I defined HasPrerequisite property from  
>> lecture to
>> lecture. And by making restrictions I adapt the property for each  
>> lecture
>> class.
>> My question is:
>> If a lecture has only 1 prerequisite lecture, I define a hasValue
>> restriction. Right?
>> But if a lecture has many prerequisites I can't use hasValue with
>> anding(intersecting) them. A person can take a lecture if the person  
>> passed
>> all of the prerequisite lectures. How can I define such a  
>> restriction? Like
>> "LectureA HasPrerequisite (LectureX and LectureY and LectureZ)"
>>
>> By making hasValue restrictions for all of the prerequisites one by  
>> one.
>> Or and'ing them and using maybe allValuesFrom restriction.
> 
> Well, this is a bit of a tricky modeling situation.
> 
> But first, let's be clear about how some of the different restrictions  
> work.
> 
> hasValue takes an Individual as its argument and means that the  
> restricted property must have that individual as one of its fillers.
> 
> someValueFrom takes a Class as its argument and means that the  
> restricted property must have at least one filler from that class.
> 
> Generally, unless you want to create OWL-Full ontologies, you will not  
> be using hasValue with classes.  You would generally be using  
> someValuesFrom instead.
> 
> But some of that depends on what other choices you make in your  
> modeling.  In other words, it partly depends on what your domain of  
> discourse is, and that is where some of the choices get a bit tricky.
> 
> OPTION 1.  Make each lecture an individual.  In this case you will be  
> treating each lecture as an individual rather than as a class of  
> lectures.  It will also mean that you can't distinguish between a  
> lecture given in 2006 and one given in 2008, since there will only be  
> one individual for each lecture.
> 
> If you go this route, then you can use hasValue to set the values of  
> the pre-requisites.   You would have a separate hasValue restriction  
> for each pre-requisite.  You can have multiple such restrictions.    
> Students can take these lectures, but you won't be able to distinguish  
> among lectures taken in different years or semesters.   You also would  
> not be able to tell if two students took the lecture at the same  
> time.   In effect, you will have treated the lecture class as the  
> concrete domain object that you are talking about.
> 
> But this is still problematic because pre-requisites are not  
> restrictions on the lectures, but rather on the students who take the  
> lectures.  See discussion below.
> 
> OPTION 2.  Make each lecture a class.  The individuals of these  
> classes can be particular instances of the lectures happening, and  
> thus have particular instructors, students, times, etc.  Pre- 
> requisites could perhaps then be modeled using someValuesFrom type  
> restrictions.
> 
> But this approach is problematic because the pre-requisites are NOT  
> actually restrictions on the lectures themselves, but rather  
> restrictions on the students who are taking the lectures.  So one  
> would need to use a more complicated formulation to place the  
> restriction on the student.  I think this may be the most accurate  
> model, but the formulation makes the pre-requisites be an implicit  
> model, which would make it difficult to answer the question "What are  
> the pre-requisites of course X?".  That is because one would otherwise  
> require a higher order logic to represent the mapping rule.
> 
> This relies on the presence of some additional properties:
>      takesCourse  (Student -> Lecture)
>      hasStudent   (Lecture -> Student), inverse of takesCourse
> 
> To say that Lecture1 has Lecture2 as a pre-requisite, one could then  
> write:
>      Lecture1 <=> hasStudent allValuesFrom (Student and takesCourse  
> someValuesFrom Lecture2)
> 
> In other words, students that are taking (an instance of) Lecture1  
> must belong to the class of students how have taken (an instance of)  
> Lecture2.  Now this can be extended to a requirement of having passed  
> the class by just changing the "takesCourse" property to  
> "passesCourse" in the restriction.
> 
> This extends to multiple pre-requisites by just adding additional  
> someValuesFrom clauses:
> 
>     LectureA <=> hasStudent allValuesFrom (Student
>                                            and passesCourse  
> someValuesFrom LectureX
>                                            and passesCourse  
> someValuesFrom LectureY
>                                            and passesCourse  
> someValuesFrom LectureZ)
> 
> This gives a good structural model, although as noted above, it would  
> not be easy to query the ontology to find the restrictions from the  
> someValues from clauses.
> 
> 
> OPTION 3.  Make the pre-requisites annotations of the lecture classes  
> rather than restrictions.  In this case you would have to build your  
> own inference procedure to check for consistency, because annotations  
> don't get enforced by the classification reasoner the way restrictions  
> do.  Also, not all systems are happy having classes as values of  
> annotation properties.  Since you have your own interpretation of the  
> annotation, you can assign the appropriate semantics to the  
> interpretation and enforcement of the pre-requisites property.
> 
> OPTION 4.  Use classes as fillers of the pre-requisites property.   
> This puts you into OWL-Full and still doesn't solve the issue of the  
> student being the one restricted.
> 
> 
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