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Knowledge Representation. Structured Objects. Structural knowledge is important e.g. viral meningitis is meningitis Representation is analogous to graphs or records. Motivation: Grouping of knowledge intuitively All knowledge about an entity is stored together

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Knowledge representation l.jpg

Knowledge Representation

Structured Objects


Slide2 l.jpg

  • Structural knowledge is important

    • e.g. viral meningitis is meningitis

  • Representation is analogous to graphs or records.

  • Motivation:

    • Grouping of knowledge intuitively

      • All knowledge about an entity is stored together

    • Having intuitive access paths

  • Inferencing via:

    • Generalisations of situations

    • Properties and interrelationships

    • Inheritance of properties

      • Same benefits of object-oriented design and programming e.g. Saves on storage


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Semantic networks l.jpg
Semantic Networks

  • Concentrate on categories of objects and the relations between them

BIRTHDAY-PARTY

ELEMENT-OF

JOHN’S-B-P

food

date

CAKE

AUGUST-3

guest

place

JOHN’S-B-P

MARY


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Subset

LinkType SemanticsExample

A B A  B Cats  Mammals

A B A  B Bill  Cats

A B R(A,B) Bill 12

A B x x  A  R(x,B) Birds 2

A B x  y x  A  Birds Birds

y  B  R(x,y)

Member

R

Age

R

Legs

Parent

R


Example l.jpg
Example:

Person

member

member (is-a)

profession

Mary

lecturer

married-to

Joe

profession

engineer

lives-in

lives-in

is-a

Kingston

city


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Characteristics of Semantic Nets

  • Limited Expressiveness

    • Cannot express negation or disjunction

    • Quantification: Complex, using partitioned nets

  • Simple and easy to understand.

  • Syntax is not clear and consistent.

  • Semantics are intuitive and dependent on implementation

  • Inheritance is captured

  • Inferencing

    • Intersection Search

      • e.g. “What is the relationship between Joe and Mary?”


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Frames

  • A data structure representing a stereotype situation.

    • The pulling together of procedural and declarative knowledge.

  • Has slot names and slot fillers

  • Usually arranged in a hierarchy

    • Frames lower down inherit slot fillers from frames higher up.

    • Properties high up are fixed

    • Properties with values lower down overwrite information higher up


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Animals

Alive:

T

Flies:

F

Subset

Subset

Birds

Mammals

Legs:

2

Legs:

4

Flies:

T

Subset

Subset

Subset

Penguins

Cats

Bats

Legs:

2

Flies:

F

Flies:

T

Member

Member

Member

Opus

Bill

Pat

Name: Pat

Name: Opus

Name: Bill

Friend:

Friend:


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Types of Slots

  • Attribute-value slots

    • Primitive data types

      <name Jumbo>

    • Pointers to other frames

      <owner e56>

  • Attribute slots with value restrictions

    <owner (a person)>

    <mother (an elephant with <sex female>) >

  • Object hierarchy slots

    • super-class/subclass slots

    • member-of/instance slots

  • Procedure slots

    • Used for calculations

      • instead of storing the value e.g. salary

    • Used to propagate changes when a slot value is changed


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  • Example:

    < e1 <member-of american-person

    dog-owner>

    <name “Mickey Mouse”>

    <address “Disneyland”>

    <owns e3>

    <personality unpleasant> >

    < e3 <member-of dog>

    <name “Pluto”>

    <owned-by e1> >

    < dog-owner <superclass person>

    <owns (a dog) >

    <must-have (a dog-licence) >

    <personality pleasant> >

    <dog <superclass pet

    carnivore >

    < address (Get address of owned-by) >


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Reasoning in Frame Systems

  • Matching

    • Find a matching frame

    • Difficult because an instance frame may have values from several class frames.

    • Potential inefficiency

  • Inheritance

    • If the value of the slot is not found in the instance frame, search up the hierarchy.

      • E.g. Does Bill fly? No

    • Allows for Default Reasoning

      • e.g. Does Opus fly?

        • No, since instance value overrides class value.


Slide13 l.jpg

Bird

Move: Fly

Ostrich

Move: Walk,

Not fly

Cartoon Bird

Tweety

  • Depends on the path taken for the search.

  • Strategy 1: Use path length (BFS)

    • Tweety does not fly.


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    Bird reasoning.

    Move: Fly

    IS A

    • Problem with BFS: Does Tweety fly?

    IS A

    Ostrich

    Move: Walk,

    Not fly

    Cartoon Bird

    IS A

    Plumed Ostrich

    IS A

    White-Plumed

    Ostrich

    instance

    instance

    Tweety

    • Strategy 2: Use Inferential Distance

      • Tweety does not fly.


    Slide15 l.jpg

    Republican reasoning.

    Pacifist : false

    • Can still get a contradiction

    IS A

    Quaker

    Pacifist : true

    Conservative-Republican

    Instance

    Instance

    Dick

    Pacifist : ?


    Advantages l.jpg
    Advantages reasoning.

    • More knowledge about the nature of the entities involved.

      • More than logic or production rules

    • Can represent highly structured knowledge

    • Easy

      • To maintain

      • To add new objects

    • Default Reasoning Possible

      • “The drawing of plausible inferences on the basis of less than conclusive evidence in the absence of evidence to the contrary.” (Moore 1985)


    Disadvantages l.jpg
    Disadvantages reasoning.

    • Precise notion of meaning is absent

      • Translation work has been done, but has difficulties with default reasoning and procedural attachment.

    • Hard to represent rules (p,q  r)


    Problems l.jpg
    Problems reasoning.

    • Draw a semantic net for the following information: Tweety is a canary who is a bird and all birds are animals. All animals breathe, typically birds have wings and their method of travel is by flying. A penguin is a bird whose method of travel is by walking. Discus how the queries “How does Tweety travel?”, “Does Tweety have wings?” and “What is the link between “Tweety and Penguins?” will be executed.


    Slide19 l.jpg

    Birds are usually, covered with feathers, fly and reproduce by laying eggs. There are three groups of birds, flightless birds, songbirds and scavengers. Flightless birds do not fly. Song birds eat bugs and seeds while scavengers eat meat. Sparrows and canaries are both flightless birds, the difference between them being that canaries are found in tropical countries, while sparrows are found in North America. Penguins are flightless birds; they eat fish and are found in the South Pole. Opus, Tweety and Beaky are all birds; Opus is a penguin, Tweety is a canary and Beaky is a rather unusual bird as he is a mix between a penguin and a canary.

    Represent the information above using frames. What would be the results of the following queries?

    • “How does Opus reproduce?”

    • “Does Beaky fly?”


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