‫بسم هللا الرحمن الرحيم‬
Umm Al-Qura University
Health Sciences College at Al-Leith
Department of Public Health
Lecture (4)
Probability Theory 
Objectives:
1/ Define basics of probability theory.
2/ Give an Example of probability theory.
Definition: Probability 
The probability of an event is the
proportion of
times that the event occurs in a large 
number of trials of the experiment.
It is the “long-run relative frequency of 
the event.”
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Definition
• Experiment: Draw a card from a standard
deck of 52.
• Sample space: The set of all possible distinct
outcomes, S (e.g., 52 cards).
• Elementary event or sample point: a
member of the sample space. (e.g., the ace
of hearts).
• Event (or event class): any set of elementary
events. e.g., Suit (Hearts), Color (Red), or
Number (Ace).
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Example
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Tossing a Coin
When a coin is tossed, there are two possible outcomes:

heads (H) or

text (T)
We say that the probability of the coin landing H is ½.
And the probability of the coin landing T is ½.
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Throwing Dice
When a single die is thrown, there are six possible
outcomes: 1, 2, 3, 4, 5, 6.
The probability of any one of them is 1/6.
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Example: the chances of rolling a "4" with a die
Number of ways it can happen: 1 (there is only 1 face
with a "4" on it)
Total number of outcomes: 6 (there are 6 faces
altogether)
So the probability =
1
6
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Example: there are 5 marbles in a bag: 4 are blue,
and 1 is red. What is the probability that a blue
marble gets picked?
Number of ways it can happen: 4 (there are 4 blues)
Total number of outcomes: 5 (there are 5 marbles in
total)
So the probability =
4
5
= 0.8
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Example: toss a coin 100 times, how many
Heads will come up?
Probability says that heads have a ½ chance, so
we can expect 50 Heads.
But when we actually try it we might get 48
heads, or 55 heads ... or anything really, but in
most cases it will be a number near 50.
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Example: choosing a card from a deck
There are 52 cards in a deck (not including Jokers)
So the Sample Space is all 52 possible cards:
{Ace of Hearts, 2 of Hearts, etc... }
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Example: Alex wants to see how many times a "double" comes up when
throwing 2 dice.
Each time Alex throws the 2 dice is an Experiment.
It is an Experiment because the result is uncertain.
The Event Alex is looking for is a "double", where both dice have the same
number. It is made up of these 6 Sample Points:
{1,1} {2,2} {3,3} {4,4} {5,5} and {6,6}
The Sample Space is all possible outcomes (36 Sample Points):
{1,1} {1,2} {1,3} {1,4} ... {6,3} {6,4} {6,5} {6,6}
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The diagram shows a spinner made up of a piece of card in the shape of a
regular pentagon, with a toothpick pushed through its center. The five triangles
are numbered from 1 to 5.
The spinner is spun until it lands on one of the five edges of the pentagon. What
is the probability that the number it lands on is odd?
3/5
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Each of the letters of the word MISSISSIPPI are written on separate pieces of
paper that are then folded, put in a hat, and mixed thoroughly.
One piece of paper is chosen (without looking) from the hat. What is the
probability it is an I?
4/11
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A card is chosen at random from a deck of 52 playing cards.
There are 4 Queens and 4 Kings in a deck of playing cards.
What is the probability the card chosen is a Queen or a King?
52/4 +8/4
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A committee of three is chosen from five councilors
- Adams, Burke, Cobb, Dilby and Evans.
What is the probability Burke is on the committee?
But all
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There are 10 counters in a bag: 3 are red, 2 are blue and 5 are green.
The contents of the bag are shaken before Maxine randomly chooses
one counter from the bag.
What is the probability that she doesn't pick a red counter?
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Sarah goes to her local pizza parlor and orders a pizza. She can
choose either a large or a medium pizza, has a choice of seven
different toppings, and can have three different choices of crust.
12
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You are buying a new car
There are 2 body styles:
sedan or hatchback
There are 5 colors available:
There are 3 models:  GL (standard model),

SS (sports model with bigger engine)

SL (luxury model with leather seats)
How many total choices?
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You can count the choices, or just do the simple calculation:
Total Choices = 2 × 5 × 3 = 30
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Example: You are buying a new car ... but ...
the salesman says "You can't choose black
for the hatchback" ... well then things
change!
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You now have only 27 choices
Because your choices are not independent of each other.
But you can still make your life easier with this calculation:
Choices = 5×3 + 4×3 = 15 + 12 = 27
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Ben can take any one of three routes from school (S) to the town center (T), and
can then take five possible routes from the town center to his home (H). He
doesn't retrace his steps.
How many different possible ways can Ben walk home from school?
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Example:
You sell sandwiches. 70% of people choose chicken, the rest
choose pork.
What is the probability of selling 2 chicken sandwiches to the
next 3 customers?
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The "Two Chicken" cases are highlighted.
0.147 + 0.147 + 0.147 = 0.441
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3 boxes
Box A contain 10 led light but 4 it bad .
Box B contain 6 led light but 1 it bad .
Box C contain 10 led light but 3 it bad .
What is the probability the chosen is a one led light but bad ? And What is the
probability the chosen is a one led light bad from box B?
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1
3
1
3
1
3
A
C
B
6
10
P (bad) =
𝟏 𝟒
( )
𝟑 𝟏𝟎
P (good) =
0.7
+
𝟏 𝟔
( )
𝟑 𝟏𝟎
𝟏 𝟏
( )
𝟑 𝟔
+
4
10
𝟏 𝟑
( )
𝟑 𝟖
+
𝟏 𝟓
( )
𝟑 𝟔
+
𝟏 𝟏
5
6
1
6
5
8
3
8
= 0.3
𝟏 𝟓
( )
𝟑 𝟖
= 0.7 OR q=1-p = 1- 0.3 =
P( box B but bad )= ( ) = 0.05
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A box of five balls pulled a ball randomly and then
restored. Calculate the sample space?
sample space = 5*5 = 25
1
2
3
4
5
1
(1,1)
(2,1)
(3,1)
(4,1)
(5,1)
2
(1,2)
(2,2)
(3,2)
(4,2)
(5,2)
3
(1,3)
(2,3)
(3,3)
(4,3)
(5,3)
4
5
(1,4) (1,5)
(2,4) (2,5)
(3,4) (3,5)
(4,4) (4,5)
(5,4) (5,5)
= 25 pairs
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if the probability of occurrence of the event A = 0.3 and the
occurrence of the event A and B = 0.4 What is the
probability event occurs B ?
P (A,B)= P(A) + P(B)
0.4= 0.3 + P(B)
P(B)= 0.4 - .03 = 0.1
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Ball pulled randomly from a unbiased box contains six red
balls and four white balls and five blue.
1- Find the sample space .
2- what is the probability that be red ?
3- what is the probability that be white ?
4- what is the probability that be blue ?
5- what is the possibility that is not blue?
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Sample space = 6+4+5 = 15
P (red) =
𝟐
𝟓
P(white) =
P(blue) =
𝟒
𝟏𝟓
𝟏
𝟑
P( not red)= 1- p (red) =
𝟑
𝟓
OR p( not red ) = p(white + blue) = p (white) + p(blue) =
P(red and blue ) = p (red) + p(blue) =
𝟑
𝟓
𝟐
𝟑
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Factory contains three machines product 50% and 30% and 20%
respectively and the damaged 3% and 4% and 5%, respectively.
What is the probability of selecting a random sample of factory
production to be damaged ?
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p(damaged) = 0.5(0.03)+ 0.3(0.04) + 0.2(0.05) =
0.37
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The End
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