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# Probabilistic Reasoning; Network-based reasoning - PowerPoint PPT Presentation

Probabilistic Reasoning; Network-based reasoning. COMPSCI 276 Fall 2007. Class Description. Instructor: Rina Dechter Days: Monday & Wednesday Time: 2:00 - 3:20 pm Class page: http://www.ics.uci.edu/~dechter/ics-275b/Fall-2007/. Why uncertainty. Summary of exceptions

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### Probabilistic Reasoning;Network-based reasoning

COMPSCI 276

Fall 2007

• Instructor: Rina Dechter

• Days: Monday & Wednesday

• Time: 2:00 - 3:20 pm

• Class page: http://www.ics.uci.edu/~dechter/ics-275b/Fall-2007/

• Summary of exceptions

• Birds fly, smoke means fire (cannot enumerate all exceptions.

• Why is it difficult?

• Exception combines in intricate ways

• e.g., we cannot tell from formulas how exceptions to rules interact:

AC

BC

---------

A and B - C

True

propositions

Uncertain

propositions

Q: Does T fly?

P(Q)?

Logic?....but how we handle exceptions

Probability: astronomical

• Knowledge obtained from people is almost always loaded with uncertainty

• Most rules have exceptions which one cannot afford to enumerate

• Antecedent conditions are ambiguously defined or hard to satisfy precisely

• First-generation expert systems combined uncertainties according to simple and uniform principle

• Lead to unpredictable and counterintuitive results

• Early days: logicist, new-calculist, neo-probabilist

• Extensional (e.g., Mycin, Shortliffe, 1976) certainty factors attached to rules and combine in different ways.

• Intensional, semantic-based, probabilities are attached to set of worlds.

AB: m

P(A|B) = m

A

x

If A then C (x)

If B then C (y)

If C then D (z)

z

C

D

y

B

1.Parallel Combination:

CF(C) = x+y-xy, if x,y>0

CF(C) = (x+y)/(1-min(x,y)), x,y have different sign

CF( C) = x+y+xy, if x,y<0

2. Series combination…

3.Conjunction, negation

Computational desire : locality, detachment, modularity

Burglery

Phone

call

Alarm

Earthquake

AB

A more credible

------------------

B more credible

IF Alarm  Burglery

But B is less credible

Rule from effect to causes

Extensional

Intensional

A and BC

(m+n-mn)

• Claim: the basic steps invoked while people query and update their knowledge corresponds to mental tracings of pre-established links in dependency graphs

• Claim: the degree to which an explanation mirrors these tracings determines whether it is psychologically meaningful.

P(C|S)

P(B|S)

• C B D=0 D=1

• 0 0 0.1 0.9

• 0 1 0.7 0.3

• 1 0 0.8 0.2

• 1 1 0.9 0.1

CPD:

P(X|C,S)

P(D|C,B)

Conditional Independencies

Efficient Representation

Bayesian Networks: Representation

Smoking

lung Cancer

Bronchitis

X-ray

Dyspnoea

P(S, C, B, X, D)

= P(S) P(C|S) P(B|S) P(X|C,S) P(D|C,B)

• Pearl Chapter 3

• (Read chapter 2 for background and refresher)

• The traditional definition of independence uses equality of numerical quantities as in P(x,y)=P(x)P(y)

• People can easily and confidently detect dependencies, even though they may not be able to provide precise numerical estimates of probabilities.

• The notion of relevance and dependence are far more basic to human reasoning than the numerical values attached to probabilistic judgements.

• Should allow assertions about dependency relationships to be expressed qualitatively, directly and explicitly.

• Once asserted, these dependency relationships should remain a part of the representation scheme, impervious to variations in numerical inputs.

The Qualitative Notion of Depedence(continue)

• Information about dependencies is essential in reasoning

• If we have acquired a body of knowledge K and now wish to assess the truth of proposition A, it is important to know whether it is worthwhile to consult another proposition B, which is not in K.

• How can we encode relevance information in a symbolic system?

• The number of (A,B,K) combinations is astronomical.

• Acquisition of new facts may destroy existing dependencies as well as create new ones (e.g.,age, hight,reading ability, or ground wet,rain,sprinkler)

• The first kind of change is called “normal” . The second will be called “induced”.

• Irrelevance is denoted: P(A|K,B)=P(A|K)

• Dependency relationships are qualitative and can be logical

• The nodes represent propositional variables and the arcs represent local dependencies among conceptually related propositions.

• Explicitness, stability

• Graph concepts are entrenched in our language (e.g., “thread of thoughts”, “lines of reasoning”, “connected ideas”)

• One wonders if people can reason any other way except by tracing links and arrows and paths in some mental representation of concepts and relations.

• What types of dependencies and independencies are deducible from the topological properties of a graph?

• For a given probability distribution P and any three variables X,Y,Z,it is straightforward to verify whether knowing Z renders X independent of Y, but P does not dictates which variables should be regarded as neighbors.

• Some useful properties of dependencies and relevancies cannot be represented graphically.

• Allow deriving conjectures about independencies that are clearer

• Axioms serve as inference rules

• Can capture the principal differences between various notions of relevance or independence