Here we have said that (1) correct classifications cost nothing, (2) calling an
abnormal pixel normal has a cost of 4, (3) calling a normal pixel abnormal has
a cost of 2, and (4) the four remaining errors have unit cost. Note that the actual
cost values are unitless, and their values are relevant only in relation to one another.
Given a cost matrix, we can set up the Bayes classifier to minimize its long-
term cost of operation. The Bayes risk for assignment to a particular class is the
cost of each outcome times the likelihood of that outcome, summed over all
possible assignments to that class. It can be computed, for the unknown pixel of
Eq. 11.27, as
R
i
¼
X
3
j¼1
C
i, j
L
j
R ¼
0:0883
0:0009
0:1762
2
4
3
5
(11:32)
In this example, the unknown pixel would be assigned ...