Chapter 4 Section 5.ppt

Section 4.5-1
Copyright © 2014, 2012, 2010 Pearson Education, Inc.
Lecture Slides
Elementary Statistics
Twelfth Edition
and the Triola Statistics Series
by Mario F. Triola

Section 4.5-2
Chapter 4
Probability
4-1 Review and Preview
4-2 Basic Concepts of Probability
4-3 Addition Rule
4-4 Multiplication Rule: Basics
4-5 Multiplication Rule: Complements and
Conditional Probability
4-6 Counting
4-7 Probabilities Through Simulations
4-8 Bayes’ Theorem

Section 4.5-3
Key Concepts
Probability of “at least one”:
Find the probability that among several trials, we
get at least one of some specified event.
Conditional probability:
Find the probability of an event when we have
additional information that some other event has
already occurred.

Section 4.5-4
Complements: The Probability
of “At Least One”
 The complement of getting at least one item of a
particular type is that you get no items of that
type.
 “At least one” is equivalent to “one or more.”

Section 4.5-5
Finding the Probability
of “At Least One”
To find the probability of at least one of
something, calculate the probability of
none and then subtract that result from 1.
That is,
P(at least one) = 1 – P(none).

Section 4.5-6
Example
Topford supplies X-Data DVDs in lots of 50, and
they have a reported defect rate of 0.5% so the
probability of a disk being defective is 0.005. It
follows that the probability of a disk being good is
0.995.
What is the probability of getting at least one
defective disk in a lot of 50?

Section 4.5-7
Example – continued
What is the probability of getting at least one
defective disk in a lot of 50?
 
 
 
50
at least 1 defective disk in 50
1 all 50 disks are good
1 0.995
1 0.778 0.222
P
P

 
 
 

Section 4.5-8
A conditional probability of an event is a
probability obtained with the additional
information that some other event has already
occurred. denotes the conditional
probability of event B occurring, given that
event A has already occurred, and it can be
found by dividing the probability of events A
and B both occurring by the probability of
event A:
( | )
P B A
( and )
( | )
( )

P A B
P B A
P A

Section 4.5-9
Intuitive Approach to
The conditional probability of B given A can
be found by assuming that event A has
occurred and then calculating the probability
that event B will occur.

Section 4.5-10
Example
Refer to the table to find the probability that a
subject actually uses drugs, given that he or she
had a positive test result.
Positive Drug Test Negative Drug Test
Subject Uses Drugs 44 (True Positive) 6 (False Negative)
Subject Does
Not Use Drugs
90 (False Positive) 860 (True Negative)

Section 4.5-11
Example - continued
Positive Drug Test Negative Drug Test
Subject Uses Drugs 44 (True Positive) 6 (False Negative)
Subject Does
Not Use Drugs
90 (False Positive) 860 (True Negative)
 
 
 
subject uses drugs | subject tests positive
subject uses drugs and subject tests positive
subject tests positive
44
44
1000 0.328
134 134
1000
P
P
P


 

Section 4.5-12
Confusion of the Inverse
To incorrectly believe that and
are the same, or to incorrectly use
one value for the other, is often called
confusion of the inverse.
( | )
P B A
( | )
P A B

Chapter 4 Section 5.ppt

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