Chapter 12 — Exam Question 5
Draw a simple error matrix, indicate what is on the axes and explain how the overall
accuracy is calculated.
Answer
reference→
A
B
C
error of
classifier’s
class↓
comission
(user’s) accuracy
a
4
1
0
1/5=20%
4/5=80%
b
0
3
2
2/5=40%
3/5=60%
c
2
0
1
1/3=33%
2/3=66%
error of emission
2/6=33%
1/4=25%
2/3=66%
trainer’s (producer’s)
4/6=66%
3/4=75%
1/3=33%
accuracy
Overall accuracy A is obtained by dividing the sum of the correctly classified
pixels (those along the main diagonal) by the total number of pixels in the image
(the sum of all pixels in the matrix, on and off the main diagonal). In our case it
is given by
A =
8/13 = 61.5%