SQQS1013-CHP03 r1
SQQS1013-CHP03 r1
SQQS1013-CHP03 r1
2. An Outcome
- The result of a single trial of a probability experiment.
3. A Sample Space
- The set of all possible outcomes of a probability experiment.
- Some sample spaces for various probability experiments are shown below
Example 1
Find the sample space for rolling two dice.
Die 2
Die1
1 2 3 4 5 6
Example 2
Find the sample space for the gender of the children if a family has three
children. Use B for boy and G for girl.
Solution:
There are two genders, male and female, and each child could be of
either gender. Hence, there are eight possibilities.
4. A Tree Diagram
Example 3
Use a tree diagram to find the sample space for the gender of three children in a
family.
B B GBB
G
G GBG
G B GGB
G GGG
Example 4
You are at a carnival. One of the carnival games involves asking you to pick a
door and then pick a curtain behind the door. There are 3 doors and 4
curtains behind each door. Use a tree diagram to find the sample spaces for
all the possible choices.
A 1, A
B 1, B
1 C 1, C
D 1, D
A 2, A
2 B 2, B
C 2, C
D 2, D
A 3, A
3 B 3, B
C 3, C
D 3, D
5. Venn Diagrams:
- developed by John Venn and are used in set theory and symbolic logic.
- have been adapted to probability theory.
- A picture (a closed geometric shape such as a rectangle, a square, or a circle)
that depicts all the possible outcomes for an experiment.
- The symbol represents the union of two events and P(A B) corresponds
to A OR B.
- The symbol represents the intersection of two events and P(A B)
corresponds to A AND B.
Venn diagram representing two events; Venn diagram representing three events;
A and B A, B and C
6. An Event
- Consists of a set of outcomes of a probability experiment.
- An event can be :
a) Simple event – the outcome that is observed in a single
experiment
- an event with one outcome
e.g: If a die is rolled once and a 6 shows. There is
only 1 outcome as a result of a single trial.
b) Compound event – an even with more than one outcome.
e.g : The event of getting an odd number when a die
is rolled since it consists of three outcomes or three
simple events.
1. Classical Probability
- Uses sample spaces to determine the probability that an event will happen.
- Assumes that all outcomes in the sample space are equally likely to occur
which means that all the events have the same probability of occurrence.
- The probability of any event E is:
Number of outcomes in E
Total number of outcomes in the sample space
Or denoted as,
n( E )
P ( E )= n ( S )
e.g.: When a single die is rolled, each outcome has the same probability of
occurring. Since there are six outcomes, each outcome has a probability of
1
6.
2. Empirical Probability
- Relies on actual experience to determine the likelihood of outcomes.
- Is based on observation.
- Given a frequency distribution, the probability of an event being in a given
class is:
Example 5
Hospital records indicate that maternity patients stayed in the hospital for the
number of days shown in the following distribution:
3. Subjective Probability
- Uses a probability value based on an educated guess or estimate, employing
opinions and inexact information.
- This guess is based on the person’s experience and evaluation of a solution.
e.g.: A physician might say that, on the basis of her diagnosis, there is a 30%
chance the patient will need an operation.
Example 6
A B
A
and
B
Intersection of A and B
Union event
Let A and B be two events defined in a sample space.
The union of events A and B is the event that occurs when either A or B or both
occur.
It is denoted as A B.
Example 7
A = event that a family owns a DVD player
B = event that a family owns a digital camera
A B Shaded area
gives the union of
events A and B.
Example 8
A senior citizens centre has 300 members. Of them, 140 are male, 210 take at least
one medicine on a permanent basis and 95 are male and take at least one medicine
on a permanent basis. Draw a Venn diagram to describe,
a) the intersection of the events “male” and “take at least one medicine on
a permanent basis”.
b) the union of the events “male” and “take at least one medicine on a
permanent basis”.
c) the intersection of the events “female” and “take at least one medicine
on a permanent basis”.
d) the union of the events “female” and “take at least one medicine on a
permanent basis”.
Solution:
45 95 115 45
Example 9
Rolling a die and getting a 6, and then rolling a second die and getting a 3.
Note:
The outcome of the rolling the first die does not affect the probability
outcome of rolling the second die.
Dependent events
When the outcome or occurrence of the first event affects the outcome or
occurrence of the second event in such a way that the probability is changed, the
events are said to be dependent events.
Some examples of dependent events:
o Drawing a card from a deck, not replacing it, and then drawing a second
card.
o Selecting a ball from an urn, not replacing it, and then selecting a second
ball.
o Having high grades and getting a scholarship.
o Parking in a no-parking zone and getting a parking ticket.
1. Joint Probability
The probability of the intersection of events.
Written by either P(A B) or P(AB).
Example
(Refer to Table 1)
If one of those employees is selected at random for membership on the employee
management committee, there are 4 joint probabilities that can be defined. That
is,
c) the probability that this employee is a male and not a college graduate
d) the probability that this employee is a female and not a college graduate
2. Marginal Probability
The probability of a single event without consideration of any event.
Also called as simple probability.
Named so as they calculated in the margins of the table (divide the
corresponding totals for the row or column by the grand total).
Example
(Refer Table 1)
If one of those employees is selected at random for membership on the
employee management committee, find the probabilities for each of the
followings:
a) the chosen employee is a male
3. Conditional Probability
Often used to gauge the relationship between two events.
Conditional probability is the probability that an event will occur given
that another event has already occurred.
Written as:
P(event 2 will occur | event 1 has already occurred)
The probability of event A given event B is
Example
(Refer to Table 1)
If one of those employees is selected at random for membership on the
employee management committee, find the probabilities for each of the
followings:
a) the chosen employee is a male given that he is graduated from college
P(M | G) =
b) the chosen employee is not a college graduate given that this employee is
female
P( | F) =
Example
A person owns a collection of 30 CDs, of which 5 are country music.
a) 2 CDs are selected at random and with replacement. Find the probability
that the second CD is country music given that the first CD is country
music.
P(CM |CM) =
b) This time the selection made is without replacement. Find the probability
that the second CD is country music given that the first CD is country
music.
P(CM |CM) =
When two events A and B are mutually exclusive, the probability that A or
B will occur is
P(A) P(B)
Addition Rule 2
P(A and B)
P(A) P(B)
Example
Consider the following events when rolling a die:
Solution:
Yes, the two events are mutually exclusive since event A and event B have no
common element.
A B
2 1
4 3
6 5
Example
Determine which events are mutually exclusive and which are not when a
single die is rolled.
Example
There are 8 nurses and 5 physicians in a hospital unit; 7 nurses and 3 physicians
are females. If a staff person is selected, find the probability that the subject is a
nurse or a male.
Solution:
P(N or M) = P(N M)
=
Example
At a convention there are 7 mathematics instructors, 5 computer sciences
instructors, 3 statistics instructors, and 4 science instructors. If an instructor is
selected, find the probability of getting a science instructor or a math instructor .
Solution:
P(science instructor or math instructor)
=
Example
A grocery store employs cashiers, stock clerks and deli personnel. The
distribution of employees according to marital status is shown here.
P(clerk married) =
P(not married) =
P( A | B ) = P(A) Or P( B | A ) = P(B)
Multiplication Rule 1
ChapterExample
3: Introduction to Probability 16
SQQS1013 Elementary Statistics
A box contains 3 red balls, 2 blue balls, and 5 white balls. A ball is selected and
its colour noted. Then it is replaced. A second ball is selected and its colour
noted. Find the probability of each of these:
a) selecting two blue balls.
Example
A survey found that 68% of book buyers are 40 years or older. If two book
buyers are selected at random, find the probability that both are 40 years or
older.
P (buyer) =
On the other hand, two events, A and B are dependent when the
occurrence of the event A changes the probability of the occurrence of
event B.
When two events are dependent, another multiplication rule can be used to
find the probability.
Multiplication Rule 2
P (A B) = P(A) P( B | A )
Example
In a scientific study there are 8 tigresses, 5 of which are pregnant. If 3 are
selected at random without replacement, find the probability that:
P(PGPGPG) =
The set of outcomes in the sample space that is not included in the outcomes
of event E.
Denoted as E’ (read “E prime”)
Example
Find the complement of each event.
Answer:
Or
P( E )=1−P (E ) Or
P( E )+ P( E)=1
Chapter 3: Introduction to Probability 19
SQQS1013 Elementary Statistics
P(E) P(E)
P(S)=1 P (E )
Example
In a group of 2000 tax payers, 400 have been audited by the IRS at least once.
If one tax payer is randomly selected from this group, what is the probability of
that tax payer never been audited by the IRS?
Solution:
Let, A = the selected tax payer has been audited by the IRS at least once
A = the selected tax payer has never been audited by the IRS
The multiplication rules can be used with the complementary event rule to
simplify solving probability problems involving “at least”.
Example
In a department store there are 120 customers, 90 of whom will buy at least one
item. If 4 customers are selected at random, one by one, find the probability that
at least one of the customers will buy at least one item. Would you consider this
event likely to occur? Explain.
Solution:
Let C = at least one customer will buy at least one item
C = none of the customers will buy at least one item
P(will buy at least one item) = 90 / 120 = ¾
So, P(won’t buy any items) = 1 - 3/4 = ¼
By using the complementary event rule,
P(C )=1−P(C )
=
Yes, this event is most likely to occur (certain event) since the probability of it
occurring is almost 1.
NOTE: The following examples are based on the overall understanding of the entire
probability concepts.
Example
A random sample of 400 college students was asked if college athletes should be
paid. The following table gives a two-way classification of the responses.
Should not be
Should be paid,
paid, PAID
Total
PAID
Student athlete, SA 90 10 100
Student non-athlete, SNA 210 90 300
400
Total 300 100
a) If one student is randomly selected from these 400 students, find the probability
that this student
P(PAID) =
ii. Favours paying college athletes given that the student selected is a non-
athlete
P(PAID | SNA) =
P(SA PAID) =
P(SNA PAID ) =
b) Are the events “student athlete” and “should be paid” independent? Are they
mutually exclusive? Explain why or why not.
P(SAPAID) =
Since, P(SAPAID) P(SA) P(PAID), those two events are not independent
(dependent).
Also, since P(SAPAID) 0, those two events are not mutually exclusive.
Example
A screening test for a certain disease is prone to giving false positives or false
negatives. If a patient being tested has the disease, the probability that the test
indicates a false negative is 0.13. If the patient does not have the disease, the
probability that the test indicates a false positive is 0.10. Assume that 3% of the
patients being tested actually have the disease.
Let D = the patient has the disease
D = the patient does not have the disease
PO = the patient tests positive
NE = the patient tests negative
Suppose that one patient is chosen at random and tested. Find the probability
that:
Joint Probability
0.87 PO P(DPO)
0.03 D
0.13 NE P(DNE)
0.10 PO D
0.97
D
0.90 NE D
a) This patient does not have the disease and tests positive
P( D PO) =
c) This patient does not have the disease and tests negative
P( D NE) =
d) This patient has the disease given that he/she tests positive
P(D | PO) =
EXERCISE 1
1. For each of the following, indicate whether the type of probability involved is an
example of classical probability, empirical probability or subjective probability:
a)The next toss of a fair coin will land on head.
b)Italy will win soccer’s World Cup the next time the competition is held.
c) The sum of the faces of two dice will be 7.
d)The train taking a commuter to work will be more than 10 minutes late.
3. Refer to question 2. List all the outcomes included in each of the following events
and mention which are simple and which are compound events.
a) Both answers are correct.
b) At most one answer is wrong.
c) The first answer is correct and the second is wrong.
d) Exactly one answer is wrong.
5. 88% of American children are covered by some type of health insurance. If four
children are selected at random, what is the probability that none are covered?
6. A box of nine golf gloves contains two left-handed gloves and seven right-handed
gloves.
a) If two gloves are randomly selected from the box without replacement, what is
the probability that both gloves selected will be right-handed?
b) If three gloves are randomly selected from the box without replacement, what
is the probability that all three will be left-handed?
c) If three gloves are randomly selected from the box without replacement, what
is the probability that at least one glove will be right-handed?
7. A financial analyst estimates that the probability that the economy will experience a
recession in the next 12 months is 25%. She also believes that if the economy
encounters recession, the probability that her mutual fund will increase in value is
20%. If there is no recession, the probability that the mutual fund will increase in
value is 75%. Find the probability that the mutual fund’s value will increase.
8. A car rental agency currently has 44 cars available. 18 of which have a GPS
navigation system. One of the 44 cars is selected at random, find the probability that
this car,
a) has a GPS navigation system.
b) does not have a GPS navigation system.
Now, two cars are selected at random from these 44 cars. Find the probability that
at least one of these cars have GPS navigation system.
9. A recent study of 300 patients found that of 100 alcoholic patients, 87 had elevated
cholesterol levels, and 200 non-alcoholic patients, 43 had elevated cholesterol
levels.
a) If a patient is selected at random, find the probability that the patient is the
following,
i. an alcoholic with elevated cholesterol level.
ii. a non-alcoholic.
iii. a non-alcoholic with non-elevated cholesterol level.
b) Are the events “alcoholic” and “non-elevated cholesterol levels” independent?
Are they mutually exclusive? Explain why or why not.
10. The probability that a randomly selected student from college is a female, is 0.55
and that a student works more than 10 hours per week, is 0.62. If these two events
are independent, find the probability that a randomly selected student is a
a) male and works for more than 10 hours per week.
b) female or works for more than 10 hours per week.
11. A housing survey studied how City Sun homeowners get to work. Suppose that the
survey consisted of a sample of 1,000 homeowners and 1,000 renters.
Drives to Work Homeowner Renter
Yes 824 681
No 176 319
c) Given that the respondent drives to work, what then is the probability that he or
she is a renter?
d) Are the two events, driving to work and being a homeowner, independent?
12. Due to the devaluation which occurred in country PQR, the consumers of that
country were buying fewer products than before the devaluation. Based on a study
conducted, the results were reported as the following:
a) it is a non-confirming bottle?
b) it was filled on machine I and is a conforming bottle?
c) it was filled on machine II or is a conforming bottle?
d) suppose you know that the bottle was produced on machine I, what is the
probability that it is non-conforming?
14. Each year, ratings are compiled concerning the performance of new cars during the
first 90 days of use. Based on a study, the probability that the new car needs a
warranty repair is 0.04, the probability that the car manufactured by Country ABC is
0.60, and the probability that the new car needs a warranty repair and was
manufactured by Country ABC is 0.025.
a) What is the probability that the car needs a warranty repair given that Country
ABC manufactured it?
b) What is the probability that the car needs a warranty repair given that Country
ABC did not manufacture it?
c) Are need for a warranty repair and country manufacturing the car statistically
independent?
15. CASTWAY is a direct selling company which has 350 authorized sales agents from
all over the country. It is known that 168 of them are male. 40% of male sales
agents have permanent jobs while half of female sales agents do not have
permanent jobs.
a) Draw a tree diagram to illustrate the above events.
b) What is the probability that a randomly selected sales agent,
i. has a permanent job?
ii. is a male given that he does not have a permanent job?
EXERCISE 2
S M N
2. The organizer has organized three games during Lam’s family day. There are run
with one leg (G), fill water in the bottle (B) and tug & war (T). 40 participants had
participated in these games. Below is the Vann Diagram showing the number of
participants for every game during the family day.
S
B G
5
9 2
2a
2a 7
5
T
b) If one participant has been selected at random, find the probability the
participant;
i. Participated in fill water in the bottle game and run with one leg
game only.
ii. Participated in all games.
iii. Participated in tug & war game given he/she has participated in
run with one leg game.
3. Harmony Cultural Club has organized three competitions; singing, dancing and
acting contests. The competition has been organized at different times and each
contestant can participate in more than one contest. Below is the Venn Diagram for
100 contestants during these competitions.
singing act
5
20 12
18
2a a
15
dance
4. Xpress Link is a courier company with 300 employees with qualification levels as
shown in the Venn diagram below. Some of the employees hold more than one
qualification.
36 2k k 50
4k
a) Find the number of employees who holds a diploma and a bachelor degree
only.
b) What is the probability one employee who has been selected at random
holds;
i. qualifications other than a master degree
ii. three qualifications.
c) Are the event that an employee holds a diploma and the event that an
employee holds a master degree independent? Prove it.
5. Given P(A) = 0.3, P(B) = 0.6 and P (A B) = 0.2. Draw a Venn diagram to
represent this statement. Then, find:
a) P(B’)
b) P(A B)
c) P(B|A)
d) P(A’ B)
e) Are A and B mutually exclusive? Prove it.
6. 5% from the total radio sales at Nora’s electric shop will be returned back for
repair by the buyer due to malfunction in the first six month. Given two radios have
been sold last week,
a) Draw a tree diagram to represent the above event.
b) Find the probability that:
i. both radios will be returned back for repair
ii. none of the radios has been returned back for repair
iii. one of the radios will be returned back for repair
iv. the second radio will be returned back for repair given the first
radio had been returned for repair.
c) Are the events of returning back both the radios for repair independent?
Prove it.
Ship Type
Company Total
Cargo Passenger
R 20 20 40
S 40 20 60
T 30 40 70
Total 90 80
8. A marketing manager wants to promote a new product of his company named Osom.
He has two marketing plans which are plan A and plan B. The probability that he will
choose plan A is 1/3. The probabilities that he does not succeed to promote the
product when using plan A and plan B is 1/5 and 1/6 respectively.
9. Two shooters have been selected to represent Malaysia in USIA game. The
probability the first shooter hits the target is ½ and the probability second shooter
misses the target is 1/3. The game will start with the first shooter and followed by the
second shooter. Draw a tree diagram to represent the events. Then, find the
probability:
TUTORIAL CHAPTER 3
QUESTION 1
Nora Kindergarten would like to conduct a Sports Day. TABLE 1 shows the number of
children based on their sport’s group.
TABLE 1
Group Boy (B) Girl (G) Total
Tuah (T) 60 70 130
Jebat (J) 30 10 40
Lekiu (L) 50 20 70
Total 140 100 240
a. If a child is selected at random, what is the probability that the child is:
b. Are the events being “female” and being in“Tuah” group dependent? Prove it?
QUESTION 2
There are 100 students enrolled at the Faculty of Sciences. Courses offered are
Mathematics (M), Physics (F) and Chemistry (K).
ii. how many students do not enrolled in either Mathematics course or Physics
course?
b. Based on a (i), if the students were randomly selected, what is the probability that a
student:
ii. enrolled in Physics and Chemistry courses but do not enrolled in Mathematics
course.
Revision History
Rev. Initiated by Description of change Last revised
no.
1 Tan Wen Dee (A132 D) Amended Q. 13 of Ex. 1 to 14 Apr 2014
include the proportion of bottles
on each machine.