# Chapter 2 part 1

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```							Pattern
Classification

All materials in these slides were taken from
Pattern Classification (2nd ed) by R. O.
Duda, P. E. Hart and D. G. Stork, John Wiley
& Sons, 2000
with the permission of the authors and the
publisher
Chapter 2 (Part 1):
Bayesian Decision Theory
(Sections 2.1-2.2)

• Introduction
• Bayesian Decision Theory–Continuous Features
2

Introduction
• The sea bass/salmon example
• State of nature, prior
• State of nature is a random variable
• The catch of salmon and sea bass is equiprobable
•   P(1) = P(2) (uniform priors)

•   P(1) + P( 2) = 1 (exclusivity and exhaustivity)

Pattern Classification, Chapter 2 (Part 1)
3

• Decision rule with only the prior information
• Decide 1 if P(1) > P(2) otherwise decide 2

• Use of the class –conditional information
• P(x | 1) and P(x | 2) describe the difference in
lightness between populations of sea and salmon

Pattern Classification, Chapter 2 (Part 1)
4

Pattern Classification, Chapter 2 (Part 1)
5

• Posterior, likelihood, evidence
• P(j | x) = P(x | j)P (j) / P(x)     (Bayes formula)

• Where in case of two categories
j2
P ( x )   P ( x |  j )P (  j )
j 1

• Posterior = (Likelihood * Prior) / Evidence
Pattern Classification, Chapter 2 (Part 1)
6

Pattern Classification, Chapter 2 (Part 1)
7

•   Decision given the posterior probabilities

X is an observation for which:

if P(1 | x) > P(2 | x)    True state of nature = 1
if P(1 | x) < P(2 | x)    True state of nature = 2

Therefore:
whenever we observe a particular x, the probability of
error is :
P(error | x) = P(1 | x) if we decide 2
P(error | x) = P(2 | x) if we decide 1
Pattern Classification, Chapter 2 (Part 1)
8

• Minimizing the probability of error
• Decide 1 if P(1 | x) > P(2 | x);
otherwise decide 2

Therefore:
P(error | x) = min [P(1 | x), P(2 | x)]
(Bayes decision)

Pattern Classification, Chapter 2 (Part 1)
Bayesian Decision Theory –                                               9

Continuous Features

• Generalization of the preceding ideas
• Use of more than one feature
• Use more than two states of nature
• Allowing actions and not only decide on the state of
nature
•   Introduce a loss of function which is more general than
the probability of error

Pattern Classification, Chapter 2 (Part 1)
10

• Allowing actions other than classification primarily
allows the possibility of rejection
• Rejection in the sense of abstention
• Don’t make a decision if the alternatives are too close
• This must be tempered by the cost of indecision

• The loss function states how costly each action
taken is

Pattern Classification, Chapter 2 (Part 1)
11

Let {1, 2,…, c} be the set of c states of nature
(or “categories”)

Let {1, 2,…, a} be the set of possible actions

Let (i | j) be the loss incurred for taking

action i when the state of nature is j

Pattern Classification, Chapter 2 (Part 1)
12
Overall risk
R = Sum of all R(i | x) for i = 1,…,a and all x

Conditional risk

Minimizing R            Minimizing R(i | x) for i = 1,…, a

j c
R(  i | x )    (  i |  j )P (  j | x )
j 1

for each action i (i = 1,…,a)
Note: This is the risk specifically for observation x
Pattern Classification, Chapter 2 (Part 1)
13

Select the action i for which R(i | x) is minimum

R is minimum and R in this case is called the
Bayes risk = best performance that can be achieved!

Pattern Classification, Chapter 2 (Part 1)
14

• Two-category classification
1 : deciding 1
2 : deciding 2
ij = (i | j)
loss incurred for deciding i when the true state of nature is j

Conditional risk:

R(1 | x) = 11P(1 | x) + 12P(2 | x)
R(2 | x) = 21P(1 | x) + 22P(2 | x)

Pattern Classification, Chapter 2 (Part 1)
15

Our rule is the following:
if R(1 | x) < R(2 | x)
action 1: “decide 1” is taken

Substituting the def. of R() we have :
decide 1 if:
11 P(1 | x) + 12P(2 | x) <
21 P(1 | x) + 22P(2 | x)

and decide 2 otherwise

Pattern Classification, Chapter 2 (Part 1)
16

We can rewrite
11 P(1 | x) + 12P(2 | x) <
21 P(1 | x) + 22P(2 | x)

As
(21- 11) P(1 | x) > (12- 22) P(2 | x)

Pattern Classification, Chapter 2 (Part 1)
17

Finally, we can rewrite
(21- 11) P(1 | x) >
(12- 22) P(2 | x)

using Bayes formula and posterior probabilities to
get:
decide 1 if:

(21- 11) P(x | 1) P(1) >
(12- 22) P(x | 2) P(2)

and decide 2 otherwise
Pattern Classification, Chapter 2 (Part 1)
18

If 21 > 11 then we can express our rule as a
Likelihood ratio:

The preceding rule is equivalent to the following rule:

P ( x |  1 ) 12   22 P (  2 )
if                         .
P ( x |  2 )  21  11 P (  1 )

Then take action 1 (decide 1)
Otherwise take action 2 (decide 2)

Pattern Classification, Chapter 2 (Part 1)
19

Optimal decision property

“If the likelihood ratio exceeds a threshold value
independent of the input pattern x, we can take
optimal actions”

Pattern Classification, Chapter 2 (Part 1)
20
Exercise

Select the optimal decision where:
= {1, 2}
P(x | 1)                N(2, 0.5) (Normal distribution)
P(x | 2)                N(1.5, 0.2)

P(1) = 2/3
P(2) = 1/3
1 2
    
3 4 
Pattern Classification, Chapter 2 (Part 1)

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