Bayesian networks
AIMA2e Chapter 14
AIMA2e Chapter 14 1
Outline
You have 3 lectures that should be read in this order:
} chapter14aBBN.pdf:intro
} chapter14bBBN.pdf:inference (skip slides:8,9,11,12,[2338])
} chapter14cMSBNx.ppt:intro with MS Belief Network software
} Bayesian Belief Net.doc (extensive/detailed tutorial use only as tuto
rial/reference if you don't understand the above slides)
AIMA2e Chapter 14 2
Outline
} Syntax
} Semantics
} Constructing Bayesian Networks
AIMA2e Chapter 14 3
Bayesian networks
A Bayesian Belief Network is a directed acyclic graph where:
a set of nodes,one per random variable
a directed edges forming an acyclic graph (link
¼
\directly in°uences")
a conditional distribution for each node given its parents:
P(X
i
jParents(X
i
))
The arcs can be thought as causal relationships between variables,
but speci¯cally an arc from X to Y means that
X has direct in°uence
on our belief in Y.
Weather
Cavity
Toothache
Catch
AIMA2e Chapter 14 4
Bayesian networks
Notice that a BBN is a simple graphical notation for conditional indepen
dence assertions and hence for compact speci¯cation of full joint distributions
Conditional distributions are represented as a
conditional probability table
AIMA2e Chapter 14 5
(CPT) giving the distribution over
X
i
for each combination of parent values
.001
P(B)
.002
P(E)
Alarm
Earthquake
MaryCalls
JohnCalls
Burglary
A
P(J)
t
f
.90
.05
A P(M)
t
f
.70
.01
B
t
t
f
f
E
t
f
t
f
P(A)
.95
.29
.001
.94
AIMA2e Chapter 14 6
Example
Topology of network encodes conditional independence assertions:
Weather
Cavity
Toothache
Catch
Weather
is independent of the other variables
Toothache
and
Catch
are conditionally independent given
Cavity
AIMA2e Chapter 14 7
Example
I'm at work,neighbor John calls to say my alarm is ringing,but neighbor
Mary doesn't call.Sometimes it's set o® by minor earthquakes.Is there a
burglar?
Variables:
Burglar
,
Earthquake
,
Alarm
,
JohnCalls
,
MaryCalls
Network topology re°ects\causal"knowledge:
{ A burglar can set the alarm o®
{ An earthquake can set the alarm o®
{ The alarm can cause Mary to call
{ The alarm can cause John to call
AIMA2e Chapter 14 8
Example contd.
AIMA2e Chapter 14 9
.001
P(B)
.002
P(E)
Alarm
Earthquake
MaryCalls
JohnCalls
Burglary
A
P(J)
t
f
.90
.05
A P(M)
t
f
.70
.01
B
t
t
f
f
E
t
f
t
f
P(A)
.95
.29
.001
.94
To be more exact,we should say in the tables
P(ajB;E)
(i.e.use
a
instead
AIMA2e Chapter 14 10
of
A
because we are listing the case for
A=True
.
AIMA2e Chapter 14 11
Global semantics of Bayesian Networks
A Bayesian network de¯nes the full joint distribution
as the product of the local conditional distributions:
P(X
1
;:::;X
n
) = ¦
n
i =1
P(X
i
jParents(X
i
))
AIMA2e Chapter 14 12
Constructing Bayesian networks
Need a method such that a series of locally testable assertions of
conditional independence guarantees the required global semantics
1.Choose an ordering of variables
X
1
;:::;X
n
2.For
i
= 1 to
n
add
X
i
to the network
select parents from
X
1
;:::;X
i¡1
such that
P(X
i
jParents(X
i
)) = P(X
i
jX
1
;:::;X
i¡1
)
This choice of parents guarantees the global semantics:
P(X
1
;:::;X
n
) = ¦
n
i =1
P(X
i
jX
1
;:::;X
i¡1
)
(chain rule)
= ¦
n
i =1
P(X
i
jParents(X
i
))
(by construction)
AIMA2e Chapter 14 13
Example
Suppose we choose the ordering
M
,
J
,
A
,
B
,
E
P(JjM) = P(J)
?
AIMA2e Chapter 14 14
Example
Suppose we choose the ordering
M
,
J
,
A
,
B
,
E
P(JjM) = P(J)
?No
P(AjJ;M) = P(AjJ)
?
P(AjJ;M) = P(A)
?
AIMA2e Chapter 14 15
Example
Suppose we choose the ordering
M
,
J
,
A
,
B
,
E
P(JjM) = P(J)
?No
P(AjJ;M) = P(AjJ)
?
P(AjJ;M) = P(A)
?No
P(BjA;J;M) = P(BjA)
?
P(BjA;J;M) = P(B)
?
AIMA2e Chapter 14 16
Example
Suppose we choose the ordering
M
,
J
,
A
,
B
,
E
P(JjM) = P(J)
?No
P(AjJ;M) = P(AjJ)
?
P(AjJ;M) = P(A)
?No
P(BjA;J;M) = P(BjA)
?Yes
P(BjA;J;M) = P(B)
?No
P(EjB;A;J;M) = P(EjA)
?
P(EjB;A;J;M) = P(EjA;B)
?
AIMA2e Chapter 14 17
Example
Suppose we choose the ordering
M
,
J
,
A
,
B
,
E
P(JjM) = P(J)
?No
P(AjJ;M) = P(AjJ)
?
P(AjJ;M) = P(A)
?No
P(BjA;J;M) = P(BjA)
?Yes
P(BjA;J;M) = P(B)
?No
P(EjB;A;J;M) = P(EjA)
?No
P(EjB;A;J;M) = P(EjA;B)
?Yes
AIMA2e Chapter 14 18
Example contd.
Deciding conditional independence is hard in noncausal directions
(Causal models and conditional independence seem hardwired for humans!)
Assessing conditional probabilities is hard in noncausal directions
AIMA2e Chapter 14 19
Compactness
A CPTfor Boolean
X
i
with
k
Boolean parents has
2
k
rows for the combinations of parent values
Each row requires one number
p
for
X
i
=true
(the number for
X
i
=false
is just
1 ¡p
)
If each variable has no more than
k
parents,
the complete network requires
O(n ¢ 2
k
)
numbers
I.e.,grows linearly with
n
,vs.
O(2
n
)
for the full joint distribution
For the small burglary net,
1 +1 +4 +2 +2 =10
numbers (vs.
2
5
¡1 = 31
)
For the large burglary net,the network is less compact:
1+2+4+2+4 =13
numbers needed
AIMA2e Chapter 14 20
Global semantics of Bayesian Networks
A Bayesian network de¯nes the full joint distribution
as the product of the local conditional distributions:
P(X
1
;:::;X
n
) = ¦
n
i =1
P(X
i
jParents(X
i
))
e.g.,
P(j ^m^a ^:b ^:e)
=
AIMA2e Chapter 14 21
Global semantics of Bayesian Networks
A Bayesian network de¯nes the full joint distribution
as the product of the local conditional distributions:
P(X
1
;:::;X
n
) = ¦
n
i =1
P(X
i
jParents(X
i
))
e.g.,
P(j ^m^a ^:b ^:e)
= P(jja)P(mja)P(aj:b;:e)P(:b)P(:e)
AIMA2e Chapter 14 22
Local semantics
Local
semantics:each node is conditionally independent
of its nondescendants given its parents
Theorem:
Local semantics
,
global semantics
AIMA2e Chapter 14 23
Markov blanket
Each node is conditionally independent of all others given its
Markov blanket
:parents + children + children's parents
AIMA2e Chapter 14 24
Example:Car diagnosis
Initial evidence:car won't start
Testable variables (green),\broken,so ¯x it"variables (orange)
Hidden variables (gray) ensure sparse structure,reduce parameters
AIMA2e Chapter 14 25
Example:Car insurance
SocioEcon
Age
GoodStudent
ExtraCar
Mileage
VehicleYear
RiskAversion
SeniorTrain
DrivingSkill
MakeModel
DrivingHist
DrivQuality
Antilock
Airbag
CarValue
HomeBase
AntiTheft
Theft
OwnDamage
PropertyCost
LiabilityCost
MedicalCost
Cushioning
Ruggedness
Accident
OtherCost
OwnCost
AIMA2e Chapter 14 26
Summary
Bayes nets provide a natural representation for (causally induced)
conditional independence
Topology + CPTs = compact representation of joint distribution
Generally easy for (non)experts to construct
SOME OF THE TOPIC (more examples and how belief update is done) IS
COVERED IN Bayesian Belief Net.doc!
AIMA2e Chapter 14 27
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