2013-1 Machine Learning Lecture 03 - Andrew Moore - bayes nets for represe…

Oct 15th, 2001Copyright © 2001, Andrew W. Moore
Bayes Nets for representing
and reasoning about
uncertainty
Andrew W. Moore
Associate Professor
School of Computer Science
Carnegie Mellon University
www.cs.cmu.edu/~awm
awm@cs.cmu.edu
412-268-7599
Note to other teachers and users of
these slides. Andrew would be delighted
if you found this source material useful in
giving your own lectures. Feel free to use
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to fit your own needs. PowerPoint
originals are available. If you make use
of a significant portion of these slides in
your own lecture, please include this
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Comments and corrections gratefully
received.

Bayes Nets: Slide 2Copyright © 2001, Andrew W. Moore
What we’ll discuss
• Recall the numerous and dramatic benefits
of Joint Distributions for describing uncertain
worlds
• Reel with terror at the problem with using
Joint Distributions
• Discover how Bayes Net methodology
allows us to built Joint Distributions in
manageable chunks
• Discover there’s still a lurking problem…
• …Start to solve that problem

Why this matters
• In Andrew’s opinion, the most important
technology in the Machine Learning / AI field
to have emerged in the last 10 years.
• A clean, clear, manageable language and
methodology for expressing what you’re
certain and uncertain about
• Already, many practical applications in
medicine, factories, helpdesks:
P(this problem | these symptoms)
anomalousness of this observation
choosing next diagnostic test | these observations

Why this matters
• In Andrew’s opinion, the most important
technology in the Machine Learning / AI field
to have emerged in the last 10 years.
• A clean, clear, manageable language and
methodology for expressing what you’re
certain and uncertain about
• Already, many practical applications in
medicine, factories, helpdesks:
P(this problem | these symptoms)
anomalousness of this observation
choosing next diagnostic test | these observations
Anomaly
Detection
Inference
Active Data
Collection

Ways to deal with Uncertainty
• Three-valued logic: True / False / Maybe
• Fuzzy logic (truth values between 0 and 1)
• Non-monotonic reasoning (especially
focused on Penguin informatics)
• Dempster-Shafer theory (and an extension
known as quasi-Bayesian theory)
• Possibabilistic Logic
• Probability

Discrete Random Variables
• A is a Boolean-valued random variable if A
denotes an event, and there is some degree
of uncertainty as to whether A occurs.
• Examples
• A = The US president in 2023 will be male
• A = You wake up tomorrow with a headache
• A = You have Ebola

Probabilities
• We write P(A) as “the fraction of possible
worlds in which A is true”
• We could at this point spend 2 hours on the
philosophy of this.
• But we won’t.

Visualizing A
Event space of
all possible
worlds
Its area is 1
Worlds in which A is False
Worlds in which
A is true
P(A) = Area of
reddish oval

Interpreting the axioms
• 0 <= P(A) <= 1
• P(True) = 1
• P(False) = 0
• P(A or B) = P(A) + P(B) - P(A and B)
The area of A can’t get
any smaller than 0
And a zero area would
mean no world could
ever have A true

• 0 <= P(A) <= 1
• P(True) = 1
• P(False) = 0
The area of A can’t get
any bigger than 1
And an area of 1 would
mean all worlds will have
A true

• 0 <= P(A) <= 1
• P(True) = 1
• P(False) = 0
A
B

• 0 <= P(A) <= 1
• P(True) = 1
• P(False) = 0
A
B
P(A or B)
BP(A and B)
Simple addition and subtraction

These Axioms are Not to be
Trifled With
• There have been attempts to do different
methodologies for uncertainty
• Fuzzy Logic
• Three-valued logic
• Dempster-Shafer
• Non-monotonic reasoning
• But the axioms of probability are the only
system with this property:
If you gamble using them you can’t be unfairly exploited by
an opponent using some other system [di Finetti 1931]

Theorems from the Axioms
• 0 <= P(A) <= 1, P(True) = 1, P(False) = 0
From these we can prove:
P(not A) = P(~A) = 1-P(A)
• How?

Side Note
• I am inflicting these proofs on you for two
reasons:
1. These kind of manipulations will need to be
second nature to you if you use probabilistic
analytics in depth
2. Suffering is good for you

Another important theorem
• 0 <= P(A) <= 1, P(True) = 1, P(False) = 0
From these we can prove:
P(A) = P(A ^ B) + P(A ^ ~B)
• How?

Conditional Probability
• P(A|B) = Fraction of worlds in which B is true
that also have A true
F
H
H = “Have a headache”
F = “Coming down with Flu”
P(H) = 1/10
P(F) = 1/40
P(H|F) = 1/2
“Headaches are rare and flu
is rarer, but if you’re coming
down with ‘flu there’s a 50-
50 chance you’ll have a
headache.”

Conditional Probability
F
H
H = “Have a headache”
F = “Coming down with Flu”
P(H) = 1/10
P(F) = 1/40
P(H|F) = 1/2
P(H|F) = Fraction of flu-inflicted
worlds in which you have a
headache
= #worlds with flu and headache
------------------------------------
#worlds with flu
= Area of “H and F” region
------------------------------
Area of “F” region
= P(H ^ F)
-----------
P(F)

Definition of Conditional Probability
P(A ^ B)
P(A|B) = -----------
P(B)
Corollary: The Chain Rule
P(A ^ B) = P(A|B) P(B)

Bayes Rule
P(A ^ B) P(A|B) P(B)
P(B|A) = ----------- = ---------------
P(A) P(A)
This is Bayes Rule
Bayes, Thomas (1763) An essay
towards solving a problem in the doctrine
of chances. Philosophical Transactions
of the Royal Society of London, 53:370-
418

Using Bayes Rule to Gamble
The “Win” envelope
has a dollar and four
beads in it
$1.00
The “Lose” envelope
has three beads and
no money
Trivial question: someone draws an envelope at random and offers to
sell it to you. How much should you pay?
R R B B R B B

Using Bayes Rule to Gamble
The “Win” envelope
has a dollar and four
beads in it
$1.00
The “Lose” envelope
has three beads and
no money
Interesting question: before deciding, you are allowed to see one bead
drawn from the envelope.
Suppose it’s black: How much should you pay?
Suppose it’s red: How much should you pay?

Calculation…
$1.00

Multivalued Random Variables
• Suppose A can take on more than 2 values
• A is a random variable with arity k if it can
take on exactly one value out of {v1,v2, .. vk}
• Thus…
jivAvAP ji  if0)(
1)( 21  kvAvAvAP

An easy fact about Multivalued Random Variables:
• Using the axioms of probability…
0 <= P(A) <= 1, P(True) = 1, P(False) = 0
P(A or B) = P(A) + P(B) - P(A and B)
• And assuming that A obeys…
• It’s easy to prove that
1)( 21  kvAvAvAP
)()(
1
21 

i
j
ji vAPvAvAvAP

An easy fact about Multivalued Random Variables:
0 <= P(A) <= 1, P(True) = 1, P(False) = 0
1)( 21  kvAvAvAP
)()(
1
21 

i
j
ji vAPvAvAvAP
• And thus we can prove
1)(
1

k
j
jvAP

Another fact about Multivalued Random Variables:
0 <= P(A) <= 1, P(True) = 1, P(False) = 0
1)( 21  kvAvAvAP
)(])[(
1
21 

i
j
ji vABPvAvAvABP

Another fact about Multivalued Random Variables:
0 <= P(A) <= 1, P(True) = 1, P(False) = 0
1)( 21  kvAvAvAP
)(])[(
1
21 

i
j
ji vABPvAvAvABP
• And thus we can prove
)()(
1


k
j
jvABPBP

More General Forms of Bayes Rule
)(~)|~()()|(
)()|(
)|(
APABPAPABP
APABP
BAP


)(
)()|(
)|(
XBP
XAPXABP
XBAP




More General Forms of Bayes Rule



 An
k
kk
ii
i
vAPvABP
vAPvABP
BvAP
1
)()|(
)()|(
)|(

Useful Easy-to-prove facts
1)|()|(  BAPBAP
1)|(
1

An
k
k BvAP

The Joint Distribution
Recipe for making a joint distribution
of M variables:
Example: Boolean
variables A, B, C

of M variables:
1. Make a truth table listing all
combinations of values of your
variables (if there are M Boolean
variables then the table will have
2M rows).
Example: Boolean
variables A, B, C
A B C
0 0 0
0 0 1
0 1 0
0 1 1
1 0 0
1 0 1
1 1 0
1 1 1

of M variables:
2M rows).
2. For each combination of values,
say how probable it is.
Example: Boolean
variables A, B, C
A B C Prob
0 0 0 0.30
0 0 1 0.05
0 1 0 0.10
0 1 1 0.05
1 0 0 0.05
1 0 1 0.10
1 1 0 0.25
1 1 1 0.10

of M variables:
2M rows).
2. For each combination of values,
say how probable it is.
3. If you subscribe to the axioms of
probability, those numbers must
sum to 1.
Example: Boolean
variables A, B, C
A B C Prob
0 0 0 0.30
0 0 1 0.05
0 1 0 0.10
0 1 1 0.05
1 0 0 0.05
1 0 1 0.10
1 1 0 0.25
1 1 1 0.10
A
B
C0.05
0.25
0.10 0.050.05
0.10
0.10
0.30

Using the
Joint
Once you have the JD you
can ask for the probability of
any logical expression
involving your attribute

E
PEP
matchingrows
)row()(

Using the
Joint
P(Poor Male) = 0.4654

E
PEP
matchingrows
)row()(

Using the
Joint
P(Poor) = 0.7604

E
PEP
matchingrows
)row()(

Inference
with the
Joint





2
21
matchingrows
andmatchingrows
2
21
21
)row(
)row(
)(
)(
)|(
E
EE
P
P
EP
EEP
EEP

Inference
with the
Joint





2
21
matchingrows
andmatchingrows
2
21
21
)row(
)row(
)(
)(
)|(
E
EE
P
P
EP
EEP
EEP
P(Male | Poor) = 0.4654 / 0.7604 = 0.612

Joint distributions
• Good news
Once you have a joint
distribution, you can
ask important
questions about
stuff that involves a
lot of uncertainty
• Bad news
Impossible to create
for more than about
ten attributes
because there are
so many numbers
needed when you
build the damn thing.

Using fewer numbers
Suppose there are two events:
• M: Manuela teaches the class (otherwise it’s Andrew)
• S: It is sunny
The joint p.d.f. for these events contain four entries.
If we want to build the joint p.d.f. we’ll have to invent those
four numbers. OR WILL WE??
• We don’t have to specify with bottom level conjunctive
events such as P(~M^S) IF…
• …instead it may sometimes be more convenient for us
to specify things like: P(M), P(S).
But just P(M) and P(S) don’t derive the joint distribution. So
you can’t answer all questions.

Independence
“The sunshine levels do not depend on and do not
influence who is teaching.”
This can be specified very simply:
P(S  M) = P(S)
This is a powerful statement!
It required extra domain knowledge. A different kind
of knowledge than numerical probabilities. It needed
an understanding of causation.

Independence
From P(S  M) = P(S), the rules of probability imply: (can
you prove these?)
• P(~S  M) = P(~S)
• P(M  S) = P(M)
• P(M ^ S) = P(M) P(S)
• P(~M ^ S) = P(~M) P(S), (PM^~S) = P(M)P(~S),
P(~M^~S) = P(~M)P(~S)

Independence
From P(S  M) = P(S), the rules of probability imply: (can
you prove these?)
• P(~S  M) = P(~S)
• P(M  S) = P(M)
• P(M ^ S) = P(M) P(S)
• P(~M ^ S) = P(~M) P(S), (PM^~S) = P(M)P(~S),
P(~M^~S) = P(~M)P(~S)
And in general:
P(M=u ^ S=v) = P(M=u) P(S=v)
for each of the four combinations of
u=True/False
v=True/False

Independence
We’ve stated:
P(M) = 0.6
P(S) = 0.3
P(S  M) = P(S)
M S Prob
T T
T F
F T
F F
And since we now have the joint pdf, we can make
any queries we like.
From these statements, we can
derive the full joint pdf.

A more interesting case
• M : Manuela teaches the class
• S : It is sunny
• L : The lecturer arrives slightly late.
Assume both lecturers are sometimes delayed by bad
weather. Andrew is more likely to arrive late than Manuela.

• S : It is sunny
Let’s begin with writing down knowledge we’re happy about:
P(S  M) = P(S), P(S) = 0.3, P(M) = 0.6
Lateness is not independent of the weather and is not
independent of the lecturer.

• S : It is sunny
Let’s begin with writing down knowledge we’re happy about:
P(S  M) = P(S), P(S) = 0.3, P(M) = 0.6
Lateness is not independent of the weather and is not
independent of the lecturer.
We already know the Joint of S and M, so all we need now is
P(L  S=u, M=v)
in the 4 cases of u/v = True/False.

• S : It is sunny
P(S  M) = P(S)
P(S) = 0.3
P(M) = 0.6
P(L  M ^ S) = 0.05
P(L  M ^ ~S) = 0.1
P(L  ~M ^ S) = 0.1
P(L  ~M ^ ~S) = 0.2
Now we can derive a full joint
p.d.f. with a “mere” six numbers
instead of seven*
*Savings are larger for larger numbers of variables.

• S : It is sunny
P(S  M) = P(S)
P(S) = 0.3
P(M) = 0.6
P(L  M ^ S) = 0.05
P(L  M ^ ~S) = 0.1
P(L  ~M ^ S) = 0.1
P(L  ~M ^ ~S) = 0.2
Question: Express
P(L=x ^ M=y ^ S=z)
in terms that only need the above
expressions, where x,y and z may
each be True or False.

A bit of notation
P(S  M) = P(S)
P(S) = 0.3
P(M) = 0.6
P(L  M ^ S) = 0.05
P(L  M ^ ~S) = 0.1
P(L  ~M ^ S) = 0.1
P(L  ~M ^ ~S) = 0.2
S M
L
P(s)=0.3
P(M)=0.6
P(LM^S)=0.05
P(LM^~S)=0.1
P(L~M^S)=0.1
P(L~M^~S)=0.2

A bit of notation
P(S  M) = P(S)
P(S) = 0.3
P(M) = 0.6
P(L  M ^ S) = 0.05
P(L  M ^ ~S) = 0.1
P(L  ~M ^ S) = 0.1
P(L  ~M ^ ~S) = 0.2
S M
L
P(s)=0.3
P(M)=0.6
P(LM^S)=0.05
P(LM^~S)=0.1
P(L~M^S)=0.1
P(L~M^~S)=0.2
Read the absence of an arrow
between S and M to mean “it
would not help me predict M if I
knew the value of S”
Read the two arrows into L to
mean that if I want to know the
value of L it may help me to
know M and to know S.
Thiskindofstuffwillbe
thoroughlyformalizedlater

An even cuter trick
Suppose we have these three events:
• M : Lecture taught by Manuela
• L : Lecturer arrives late
• R : Lecture concerns robots
Suppose:
• Andrew has a higher chance of being late than Manuela.
• Andrew has a higher chance of giving robotics lectures.
What kind of independence can we find?
How about:
• P(L  M) = P(L) ?
• P(R  M) = P(R) ?
• P(L  R) = P(L) ?

Conditional independence
Once you know who the lecturer is, then whether
they arrive late doesn’t affect whether the lecture
concerns robots.
P(R  M,L) = P(R  M) and
P(R  ~M,L) = P(R  ~M)
We express this in the following way:
“R and L are conditionally independent given M”
M
L R
Given knowledge of M,
knowing anything else in
the diagram won’t help
us with L, etc.
..which is also notated
by the following
diagram.

Conditional Independence formalized
R and L are conditionally independent given M if
for all x,y,z in {T,F}:
P(R=x  M=y ^ L=z) = P(R=x  M=y)
More generally:
Let S1 and S2 and S3 be sets of variables.
Set-of-variables S1 and set-of-variables S2 are
conditionally independent given S3 if for all
assignments of values to the variables in the sets,
P(S1’s assignments  S2’s assignments & S3’s assignments)=
P(S1’s assignments  S3’s assignments)

Example:
P(R=x  M=y ^ L=z) = P(R=x  M=y)
More generally:
“Shoe-size is conditionally independent of Glove-size given
height weight and age”
means
forall s,g,h,w,a
P(ShoeSize=s|Height=h,Weight=w,Age=a)
=
P(ShoeSize=s|Height=h,Weight=w,Age=a,GloveSize=g)

Example:
P(R=x  M=y ^ L=z) = P(R=x  M=y)
More generally:
“Shoe-size is conditionally independent of Glove-size given
height weight and age”
does not mean
forall s,g,h
P(ShoeSize=s|Height=h)
=
P(ShoeSize=s|Height=h, GloveSize=g)

Conditional
independence
M
L R
We can write down P(M). And then, since we know
L is only directly influenced by M, we can write
down the values of P(LM) and P(L~M) and know
we’ve fully specified L’s behavior. Ditto for R.
P(M) = 0.6
P(L  M) = 0.085
P(L  ~M) = 0.17
P(R  M) = 0.3
P(R  ~M) = 0.6
‘R and L conditionally
independent given M’

Conditional independence
M
L R
P(M) = 0.6
P(L  M) = 0.085
P(L  ~M) = 0.17
P(R  M) = 0.3
P(R  ~M) = 0.6
Conditional Independence:
P(RM,L) = P(RM),
P(R~M,L) = P(R~M)
Again, we can obtain any member of the Joint
prob dist that we desire:
P(L=x ^ R=y ^ M=z) =

Assume five variables
T: The lecture started by 10:35
L: The lecturer arrives late
R: The lecture concerns robots
M: The lecturer is Manuela
S: It is sunny
• T only directly influenced by L (i.e. T is
conditionally independent of R,M,S given L)
• L only directly influenced by M and S (i.e. L is
conditionally independent of R given M & S)
• R only directly influenced by M (i.e. R is
conditionally independent of L,S, given M)
• M and S are independent

Making a Bayes net
S M
R
L
T
Step One: add variables.
• Just choose the variables you’d like to be included in the
net.
S: It is sunny

Making a Bayes net
S M
R
L
T
Step Two: add links.
• The link structure must be acyclic.
• If node X is given parents Q1,Q2,..Qn you are promising
that any variable that’s a non-descendent of X is
conditionally independent of X given {Q1,Q2,..Qn}
S: It is sunny

Making a Bayes net
S M
R
L
T
P(s)=0.3
P(M)=0.6
P(RM)=0.3
P(R~M)=0.6
P(TL)=0.3
P(T~L)=0.8
P(LM^S)=0.05
P(LM^~S)=0.1
P(L~M^S)=0.1
P(L~M^~S)=0.2
Step Three: add a probability table for each node.
• The table for node X must list P(X|Parent Values) for each
possible combination of parent values
S: It is sunny

Making a Bayes net
S M
R
L
T
P(s)=0.3
P(M)=0.6
P(RM)=0.3
P(R~M)=0.6
P(TL)=0.3
P(T~L)=0.8
P(LM^S)=0.05
P(LM^~S)=0.1
P(L~M^S)=0.1
P(L~M^~S)=0.2
• Two unconnected variables may still be correlated
• Each node is conditionally independent of all non-
descendants in the tree, given its parents.
• You can deduce many other conditional independence
relations from a Bayes net. See the next lecture.
S: It is sunny

Bayes Nets Formalized
A Bayes net (also called a belief network) is an
augmented directed acyclic graph, represented by
the pair V , E where:
• V is a set of vertices.
• E is a set of directed edges joining vertices. No
loops of any length are allowed.
Each vertex in V contains the following information:
• The name of a random variable
• A probability distribution table indicating how the
probability of this variable’s values depends on
all possible combinations of parental values.

Building a Bayes Net
1. Choose a set of relevant variables.
2. Choose an ordering for them
3. Assume they’re called X1 .. Xm (where X1 is the
first in the ordering, X1 is the second, etc)
4. For i = 1 to m:
1. Add the Xi node to the network
2. Set Parents(Xi ) to be a minimal subset of
{X1…Xi-1} such that we have conditional
independence of Xi and all other members of
{X1…Xi-1} given Parents(Xi )
3. Define the probability table of
P(Xi =k  Assignments of Parents(Xi ) ).

Example Bayes Net Building
Suppose we’re building a nuclear power station.
There are the following random variables:
GRL : Gauge Reads Low.
CTL : Core temperature is low.
FG : Gauge is faulty.
FA : Alarm is faulty
AS : Alarm sounds
• If alarm working properly, the alarm is meant to
sound if the gauge stops reading a low temp.
• If gauge working properly, the gauge is meant to
read the temp of the core.

Computing a Joint Entry
How to compute an entry in a joint distribution?
E.G: What is P(S ^ ~M ^ L ~R ^ T)?
S M
R
L
T
P(s)=0.3
P(M)=0.6
P(RM)=0.3
P(R~M)=0.6
P(TL)=0.3
P(T~L)=0.8
P(LM^S)=0.05
P(LM^~S)=0.1
P(L~M^S)=0.1
P(L~M^~S)=0.2

Computing with Bayes Net
P(T ^ ~R ^ L ^ ~M ^ S) =
P(T  ~R ^ L ^ ~M ^ S) * P(~R ^ L ^ ~M ^ S) =
P(T  L) * P(~R ^ L ^ ~M ^ S) =
P(T  L) * P(~R  L ^ ~M ^ S) * P(L^~M^S) =
P(T  L) * P(~R  ~M) * P(L^~M^S) =
P(T  L) * P(~R  ~M) * P(L~M^S)*P(~M^S) =
P(T  L) * P(~R  ~M) * P(L~M^S)*P(~M | S)*P(S) =
P(T  L) * P(~R  ~M) * P(L~M^S)*P(~M)*P(S).
S M
R
L
T
P(s)=0.3
P(M)=0.6
P(RM)=0.3
P(R~M)=0.6
P(TL)=0.3
P(T~L)=0.8
P(LM^S)=0.05
P(LM^~S)=0.1
P(L~M^S)=0.1
P(L~M^~S)=0.2

The general case
P(X1
=x1 ^ X2=x2 ^ ….Xn-1=xn-1 ^ Xn=xn) =
P(Xn=xn ^ Xn-1=xn-1 ^ ….X2=x2 ^ X1=x1) =
P(Xn=xn  Xn-1=xn-1 ^ ….X2=x2 ^ X1=x1) * P(Xn-1=xn-1 ^…. X2=x2 ^ X1=x1) =
P(Xn=xn  Xn-1=xn-1 ^ ….X2=x2 ^ X1=x1) * P(Xn-1=xn-1 …. X2=x2 ^ X1=x1) *
P(Xn-2=xn-2 ^…. X2=x2 ^ X1=x1) =
:
:
=
       
    







n
i
iii
n
i
iiii
XxXP
xXxXxXP
1
1
1111
ParentsofsAssignment

So any entry in joint pdf table can be computed. And so any
conditional probability can be computed.

Where are we now?
• We have a methodology for building Bayes nets.
• We don’t require exponential storage to hold our probability
table. Only exponential in the maximum number of parents
of any node.
• We can compute probabilities of any given assignment of
truth values to the variables. And we can do it in time
linear with the number of nodes.
• So we can also compute answers to any questions.
E.G. What could we do to compute P(R  T,~S)?
S M
R
L
T
P(s)=0.3
P(M)=0.6
P(RM)=0.3
P(R~M)=0.6
P(TL)=0.3
P(T~L)=0.8
P(LM^S)=0.05
P(LM^~S)=0.1
P(L~M^S)=0.1
P(L~M^~S)=0.2

Where are we now?
of any node.
S M
R
L
T
P(s)=0.3
P(M)=0.6
P(RM)=0.3
P(R~M)=0.6
P(TL)=0.3
P(T~L)=0.8
P(LM^S)=0.05
P(LM^~S)=0.1
P(L~M^S)=0.1
P(L~M^~S)=0.2
Step 1: Compute P(R ^ T ^ ~S)
Step 2: Compute P(~R ^ T ^ ~S)
Step 3: Return
P(R ^ T ^ ~S)
-------------------------------------
P(R ^ T ^ ~S)+ P(~R ^ T ^ ~S)

Where are we now?
of any node.
S M
R
L
T
P(s)=0.3
P(M)=0.6
P(RM)=0.3
P(R~M)=0.6
P(TL)=0.3
P(T~L)=0.8
P(LM^S)=0.05
P(LM^~S)=0.1
P(L~M^S)=0.1
P(L~M^~S)=0.2
Step 3: Return
P(R ^ T ^ ~S)
-------------------------------------
P(R ^ T ^ ~S)+ P(~R ^ T ^ ~S)
Sum of all the rows in the Joint
that match R ^ T ^ ~S
that match ~R ^ T ^ ~S

Where are we now?
of any node.
S M
R
L
T
P(s)=0.3
P(M)=0.6
P(RM)=0.3
P(R~M)=0.6
P(TL)=0.3
P(T~L)=0.8
P(LM^S)=0.05
P(LM^~S)=0.1
P(L~M^S)=0.1
P(L~M^~S)=0.2
Step 3: Return
P(R ^ T ^ ~S)
-------------------------------------
P(R ^ T ^ ~S)+ P(~R ^ T ^ ~S)
that match R ^ T ^ ~S
that match ~R ^ T ^ ~S
Each of these obtained by
the “computing a joint
probability entry” method of
the earlier slides
4 joint computes
4 joint computes

The good news
We can do inference. We can compute any
conditional probability:
P( Some variable  Some other variable values )





2
21
matchingentriesjoint
andmatchingentriesjoint
2
21
21
)entryjoint(
)entryjoint(
)(
)(
)|(
E
EE
P
P
EP
EEP
EEP

The good news
We can do inference. We can compute any
conditional probability:
P( Some variable  Some other variable values )





2
21
matchingentriesjoint
andmatchingentriesjoint
2
21
21
)entryjoint(
)entryjoint(
)(
)(
)|(
E
EE
P
P
EP
EEP
EEP
Suppose you have m binary-valued variables in your Bayes
Net and expression E2 mentions k variables.
How much work is the above computation?

The sad, bad news
Conditional probabilities by enumerating all matching entries
in the joint are expensive:
Exponential in the number of variables.

The sad, bad news
But perhaps there are faster ways of querying Bayes nets?
• In fact, if I ever ask you to manually do a Bayes Net
inference, you’ll find there are often many tricks to save you
time.
• So we’ve just got to program our computer to do those tricks
too, right?

The sad, bad news
But perhaps there are faster ways of querying Bayes nets?
• In fact, if I ever ask you to manually do a Bayes Net
inference, you’ll find there are often many tricks to save you
time.
• So we’ve just got to program our computer to do those tricks
too, right?
Sadder and worse news:
General querying of Bayes nets is NP-complete.

Bayes nets inference algorithms
A poly-tree is a directed acyclic graph in which no two nodes have more than one
path between them.
A poly tree Not a poly tree
(but still a legal Bayes net)
S
RL
T
L
T
MSM
R
X1
X2
X4
X3
X5
X1 X2
X3
X5
X4
• If net is a poly-tree, there is a linear-time algorithm (see a later
Andrew lecture).
• The best general-case algorithms convert a general net to a poly-
tree (often at huge expense) and calls the poly-tree algorithm.
• Another popular, practical approach (doesn’t assume poly-tree):
Stochastic Simulation.

Sampling from the Joint Distribution
It’s pretty easy to generate a set of variable-assignments at random with
the same probability as the underlying joint distribution.
How?
S M
R
L
T
P(s)=0.3
P(M)=0.6
P(RM)=0.3
P(R~M)=0.6
P(TL)=0.3
P(T~L)=0.8
P(LM^S)=0.05
P(LM^~S)=0.1
P(L~M^S)=0.1
P(L~M^~S)=0.2

Sampling from the Joint Distribution
1. Randomly choose S. S = True with prob 0.3
2. Randomly choose M. M = True with prob 0.6
3. Randomly choose L. The probability that L is true
depends on the assignments of S and M. E.G. if steps
1 and 2 had produced S=True, M=False, then
probability that L is true is 0.1
4. Randomly choose R. Probability depends on M.
5. Randomly choose T. Probability depends on L
S M
R
L
T
P(s)=0.3
P(M)=0.6
P(RM)=0.3
P(R~M)=0.6
P(TL)=0.3
P(T~L)=0.8
P(LM^S)=0.05
P(LM^~S)=0.1
P(L~M^S)=0.1
P(L~M^~S)=0.2

A general sampling algorithm
Let’s generalize the example on the previous slide to a general Bayes Net.
As in Slides 16-17 , call the variables X1 .. Xn, where Parents(Xi) must be a
subset of {X1 .. Xi-1}.
For i=1 to n:
1. Find parents, if any, of Xi. Assume n(i) parents. Call them Xp(i,1), Xp(i,2),
…Xp(i,n(i)).
2. Recall the values that those parents were randomly given: xp(i,1), xp(i,2),
…xp(i,n(i)).
3. Look up in the lookup-table for:
P(Xi=True  Xp(i,1)=xp(i,1),Xp(i,2)=xp(i,2)…Xp(i,n(i))=xp(i,n(i)))
4. Randomly set xi=True according to this probability
x1, x2,…xn are now a sample from the joint distribution of X1, X2,…Xn.

Stochastic Simulation Example
Someone wants to know P(R = True  T = True ^ S = False )
We’ll do lots of random samplings and count the number of
occurrences of the following:
• Nc : Num. samples in which T=True and S=False.
• Ns : Num. samples in which R=True, T=True and S=False.
• N : Number of random samplings
Now if N is big enough:
Nc /N is a good estimate of P(T=True and S=False).
Ns /N is a good estimate of P(R=True ,T=True , S=False).
P(RT^~S) = P(R^T^~S)/P(T^~S), so Ns / Nc can be a good
estimate of P(RT^~S).

General Stochastic Simulation
Someone wants to know P(E1  E2 )
We’ll do lots of random samplings and count the number of
occurrences of the following:
• Nc : Num. samples in which E2
• Ns : Num. samples in which E1 and E2
• N : Number of random samplings
Now if N is big enough:
Nc /N is a good estimate of P(E2).
Ns /N is a good estimate of P(E1 , E2).
P(E1  E2) = P(E1^ E2)/P(E2), so Ns / Nc can be a good estimate
of P(E1 E2).

Likelihood weighting
Problem with Stochastic Sampling:
With lots of constraints in E, or unlikely events in E, then most of the
simulations will be thrown away, (they’ll have no effect on Nc, or Ns).
Imagine we’re part way through our simulation.
In E2 we have the constraint Xi = v
We’re just about to generate a value for Xi at random. Given the values
assigned to the parents, we see that P(Xi = v  parents) = p .
Now we know that with stochastic sampling:
• we’ll generate “Xi = v” proportion p of the time, and proceed.
• And we’ll generate a different value proportion 1-p of the time, and the
simulation will be wasted.
Instead, always generate Xi = v, but weight the answer by weight “p” to
compensate.

Likelihood weighting
Set Nc :=0, Ns :=0
1. Generate a random assignment of all variables that
matches E2. This process returns a weight w.
2. Define w to be the probability that this assignment would
have been generated instead of an unmatching
assignment during its generation in the original
algorithm.Fact: w is a product of all likelihood factors
involved in the generation.
3. Nc := Nc + w
4. If our sample matches E1 then Ns := Ns + w
5. Go to 1
Again, Ns / Nc estimates P(E1  E2 )

Case Study I
Pathfinder system. (Heckerman 1991, Probabilistic Similarity Networks,
MIT Press, Cambridge MA).
• Diagnostic system for lymph-node diseases.
• 60 diseases and 100 symptoms and test-results.
• 14,000 probabilities
• Expert consulted to make net.
• 8 hours to determine variables.
• 35 hours for net topology.
• 40 hours for probability table values.
• Apparently, the experts found it quite easy to invent the causal links
and probabilities.
• Pathfinder is now outperforming the world experts in diagnosis. Being
extended to several dozen other medical domains.

Questions
• What are the strengths of probabilistic networks
compared with propositional logic?
• What are the weaknesses of probabilistic networks
compared with propositional logic?
• What are the strengths of probabilistic networks
compared with predicate logic?
• What are the weaknesses of probabilistic networks
compared with predicate logic?
• (How) could predicate logic and probabilistic
networks be combined?

What you should know
• The meanings and importance of independence
and conditional independence.
• The definition of a Bayes net.
• Computing probabilities of assignments of
variables (i.e. members of the joint p.d.f.) with a
Bayes net.
• The slow (exponential) method for computing
arbitrary, conditional probabilities.
• The stochastic simulation method and likelihood
weighting.

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