knowledge representation
DESCRIPTION
Knowledge Representation. • rule-based systems: those that use the knowledge structure if premise(s) then conclusion(s) • forward and backward chaining are the common inference strategies • logical theorem proving is the general inference paradigm - PowerPoint PPT PresentationTRANSCRIPT
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Knowledge Representation
• rule-based systems: those that use the knowledge structure
if premise(s) then conclusion(s)
• forward and backward chaining are the common inference strategies
• logical theorem proving is the general inference paradigm
- when KB is encoded as logical rules using AND, OR, NOT, implication, derive the logical truth or falsehood of some expression using the KB as a theory
- theorem proving is an active area in AI
- theoretical result: not all deductions are decideable; thus inference is a fundamentally complex and undecideable problem
• uncertainty also used
• Prolog: form conclusion :- premise, premise, ... conclusion.
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Knowledge Representation
Problems with rule-based systems
• multiple evaluation of rule (hence saving of user input)
• large rule sets: inefficient, maintainability
- 50 rule modules maximum
- if problem decomposition is difficult, perhaps change the knowledge representation (eg. add frames)
• uncertainty: choosing a technique, interpreting results
• misfit of problems to rule-based paradigm
- not all problems naturally suited
• procedural code in rules - forcing procedural execution (eg. Oops) can be messy
- Prolog hooks are a good solution
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Other techniques
i) Frames and object-oriented programming
ii) multiple contexts and truth-maintenance systems
iii) model-based representations
iv) blackboards
v) Case-based reasoning (CBR)
Rule of thumb:
When a knowledge base is difficult to understand and use,consider changing the representation.
Main goal: close the semantic gap between expert’s knowledge and computer implementation
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(i) Frames (review)
• knowledge structuring technique
• "frames": lmowledge representation exploiting structure & inheritence
"object-oriented programming": associating all data and computation with objects (= frames); computation done via message passing
• benefits:
- aids structuring of KB
- more generic rules, therefore reduce size, complexity of KB
- puts data in one area/module (a frame declaration)
- maintainability of KB
- objects, once identified, can contain characteristics, rather than spread this knowledge throughout many rules
- inference engine can more effectively use/modify the data
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Frames (cont)
• To use frames during knowledge engineering
i) decide on type of reasoning (forward, backward, other...)
ii) decide on tasks for rules and processing requirements
-iii) sketch a few rules, design a small hierarchy of frames, and test
• AI workstations have graphical frame interfaces
- permits quick definition, debugging
Problems with frames
• learning period for proper use and application
• efficiency: can be lots of execution time overhead - eg. one object change can entail 100's of subsidiary commands
• mis-use: create an inappropriate frame hierarchy/taxonomy - knowledge becomes difficult to understand - can also use too much procedural code, when declarative would be preferred
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Frames
p.250
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(ii) Multiple contexts
• context: single universe/world in which inference pertains
• some problems have different environments which result in different solutions
eg. time, season, path/direction, geographic location, ....
• multiple context expert system: performs separate inferences for different contexts, using the same ruleset
- otherwise, one inference and rule set to do the same
- complicated KB and rules, that need to reconcile different contexts - inefficient: too much inferencing and backtracking
• one can have multiple "parallel" inferences for all the contexts; then one can inspect the results for each and determine which is best alternative
- each uses identical KB rules, but may require their own environments (variable binding records, run-time databases, ....)
• akin to parallel processing instead of backtracking
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Multiple contexts
p.256
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Multiple contexts
p.257
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Multiple contexts
• Applying multiple contexts
1. selecting context structure for problem
- problem specific - often hierarchical structure are well-suited
2. Reasoning with contexts: "fit" of context to problem <---> reasoning
(i) parallel reasoning: mutually exclusive sub-problems
(ii) pruning: trim computation space (like a "cut") - else resources can be overwhelmed - dominance: once one context is known to be best, then kill others
(iii) merging: when 2 contexts have identical facts and assumptions, incorporate them together
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Truth maintenance systems
• monotonic reasoning: set of beliefs grow as inference proceeds
non-monotonic reasoning: adding contexts can reduce belief set (eg. by retracting facts)
• Truth maintenance systems (TMS): (consistency mgmt. systems)
- when one assumption changes, all conclusions from that assumption must be retracted (repeated)
• Three types of TMS:
i) Justification TMS: simplest - set of beliefs in assumption set is monotonic, never shrinks - facts are either believed or not; but not believing a fact is not the same as believing it is false
ii) Logic-based TMS - each predicate is true, false, unknown; represented in logic - contradictions are detected efficiently
iii) Assumption-based TMS - conexts used in parallel (as in previous overheads)
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TMS
p.266
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TMS
p.267
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Multiple contexts
• Searching multiple contexts: a tree of all possible inferences
(i) breadth-first: each level is generated one-by-one, fairly - complete, but resource expensive
(ii) depth-first: Prolog technique - efficient, but possibly non-terminating
(iii) controlled: more arbitrary, can move about tree as required
"general theorem proving"
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(iii) Model-based representation
• encode knowledge in a structure which naturally describes it• use: realtime monitoring and diagnosis of machinery - can reason about common and uncommon failures
(i) Static models
• graph, node & arcs
• good for representing connection paths between components/modules/...
supply input: forward chaiing supply output: backward chaining
• benefits:
- clarity: can permit visual representation - a direct simulation of object of interest - familiarity: permits better debugging of KB - maintenance: change components in model, rather than KB rules - simplification: KB is simpler - efficiency: compact representation of domain, smaller than with rules/facts
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Static models
p.289
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Static models
p.291
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Model-based representation
(ii) dynamic models:
• the state of the model changes over time, and this state change is the phenomemon of interest
• simulation language/systems
• supply inputs, system generates output, which is simulated dynamic behavior of whole system
• used when actual domain unavailable, therefore a formal model is used as a substitute
eg. nuclear power plant, earth's ozone layer, aircraft modelling, ...
• used in many AI systems (eg. KEE)
• benefit: can ascertain data that would normally have to be treated as UNKNOWN
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(iv) Blackboards
• blackboard: representation of knowledge with a high-level controlling element
• first used in Hearsay II
• components:
knowledge sources (KS) : modules with specific expertise (tests, actions) blackboard: knowledge storage and communication control: overall problem-solving strategy
• some control strategies
- event-driven: react to events
- expectation driven: predict solution based on current BB state, and act appropriately
- request driven: control given to the KS which seems appropriate to a particular request
- goal directed: control given to KS likely to solve some goal
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Blackboards
p.304
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Blackboards
•Some conditions for BB use•static knowledge sources: KS's don't change structure (BB does)
•decomposable knowledge: get a small number of complex KS's•independence of KS's: run independently without hooks to one another• - BB is a medium that buffers them•hierarchical problems: layer problem domain• - each level taken care of by one or more KS's
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Blackboard characteristics
• Diverse approaches to problem solving
– modular, independent
• Common language for interaction
– formal interfaces
• Flexible representation of information
• Efficient storage and retrieval of information
– permits KS’s to inspect BB, and retrieve info as necessary
• Organized participation
– a distributed computing problem
• Iterative problem solving
– KS’s contribute to solution incrementally
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Blackboards
• Some conditions for BB use
– static knowledge sources: KS's don't change structure (BB does)
– decomposable knowledge: get a small number of complex KS's
– independence of KS's: run independently without hooks to one another• BB is a medium that buffers them
– hierarchical problems: layer problem domain• each level taken care of by one or more KS's
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Blackboards
p.315
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Blackboards
When to use BB's
• real-time processing (speach, signal processing)
• some scheduling and planning applications
• problems which naturally decompose into separate independent tasks
• problems which are best handled procedurally
- this is also a pitfall, as BB and procedures can be applied to problems for which declarative rule-based methods would be perfectly suited
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(v) Case-based Reasoning (CBR)
• Case: operational description of a past problem and its solution• CBR idea: experts use vast amount of past experiences to derive new solutions
– their compiled knowledge often uses pattern matching on long-past experiences
• CBR records and documents a history of past cases– then attempts to match a given program with closest past case– the more the CBR system is used, the larger it’s case history becomes, and
the more effective it can be• ideal when a problem is difficult to formulate in terms of explicit rule-based
knowledge– often, the closest case matching a new problem is sufficient for correctly
diagnosing the problem• Issue: newer cases are usually more pertinent than older ones
– CBR system should account for case ages• Note: inductive inference (ID3) is one style of CBR
– each example in table is a case