Statistics: Probability

Welcome To Our
presentation on
Probability

What is Probability?
Probability is the chance that something will happen - how likely
it is that some event will happen.
Sometimes you can measure a probability with a number like
"10% chance of rain", or you can use words such as
impossible, unlikely, possible, even chance, likely and certain.
Example: "It is unlikely to rain tomorrow".

• Exhaustive Events:
The total number of all possible elementary outcomes in a random
experiment is known as ‘exhaustive events’. In other words, a set is said to be
exhaustive, when no other possibilities exists.
• Favorable Events:
The elementary outcomes which entail or favor the happening of an
event is known as ‘favorable events’ i.e., the outcomes which help in the
occurrence of that event.
• Mutually Exclusive Events:
Events are said to be ‘mutually exclusive’ if the occurrence of an event
totally prevents occurrence of all other events in a trial. In other words, two
events A and B cannot occur simultaneously.

• Equally likely or Equi-probable Events:
Outcomes are said to be ‘equally likely’ if there is no reason
to expect one outcome to occur in preference to another. i.e.,
among all exhaustive outcomes, each of them has equal chance
of occurrence.
• Complementary Events:
Let E denote occurrence of event. The complement of E
denotes the non occurrence of event E. Complement of E is
denoted by ‘Ē’.
• Independent Events:
Two or more events are said to be ‘independent’, in a
series of a trials if the outcome of one event is does not affect the
outcome of the other event or vise versa.

In other words, several events are said to be ‘dependents’ if the
occurrence of an event is affected by the occurrence of any number of
remaining events, in a series of trials.
Measurement of Probability:
There are three approaches to construct a measure of probability of
occurrence of an event. They are:
 Classical Approach,
 Frequency Approach and
 Axiomatic Approach.

Classical or Mathematical
Approach:
In this approach we assume that an experiment or trial results in
any one of many possible outcomes, each outcome being Equi-probable or
equally-likely.
Definition: If a trial results in ‘n’ exhaustive, mutually exclusive, equally
likely and independent outcomes, and if ‘m’ of them are favorable for the
happening of the event E, then the probability ‘P’ of occurrence of the event
‘E’ is given by-
P(E) =
Number of outcomes favourable to event E
Exhaustive number of outcomes
=
m
n

Empirical or Statistical Approach:
This approach is also called the ‘frequency’ approach to probability.
Here the probability is obtained by actually performing the experiment large
number of times. As the number of trials n increases, we get more accurate
result.
Definition: Consider a random experiment which is repeated large number of
times under essentially homogeneous and identical conditions. If ‘n’ denotes
the number of trials and ‘m’ denotes the number of times an event A has
occurred, then, probability of event A is the limiting value of the relative
frequency m .
n

Axiomatic Approach:
This approach was proposed by Russian Mathematician
A.N.Kolmogorov in1933.
‘Axioms’ are statements which are reasonably true and are accepted
as such, without seeking any proof.
Definition: Let S be the sample space associated with a random experiment.
Let A be any event in S. then P(A) is the probability of occurrence of A if the
following axioms are satisfied.
1. P(A)>0, where A is any event.
2. P(S)=1.
3. P(AUB) = P(A) + P(B), when event A and B are mutually exclusive.

Three types of ProbabilityThree types of Probability
1. Theoretical probability:
For theoretical reasons, we assume that all n
possible outcomes of a particular experiment are
equally likely, and we assign a probability of to each
possible outcome. Example: The theoretical
probability of rolling a 3 on a regular 6 sided die is 1/6

2. Relative frequency interpretation of probability:
How many times A occurs
How many trials
Relative Frequency is based on observation or actual
measurements.
Example: A die is rolled 100 times. The number 3 is rolled 12 times. The
relative frequency of rolling a 3 is 12/100.
3. Personal or subjective probability:
These are values (between 0 and 1 or 0 and 100%) assigned by
individuals based on how likely they think events are to occur. Example: The
probability of my being asked on a date for this weekend is 10%.
The probability of event A =


1. The probability of an event is between 0 and 1. A probability of 1 is
equivalent to 100% certainty. Probabilities can be expressed at fractions,
decimals, or percents.
0 ≤ pr(A) ≤ 1
2. The sum of the probabilities of all possible outcomes is 1 or 100%. If A, B,
and C are the only possible outcomes, then pr(A) + pr(B) + pr(C) = 1
Example: A bag contains 5 red marbles, 3 blue marbles, and 2 green
marbles. pr(red) + pr(blue) + pr(green) = 1
1
10
2
10
3
10
5
=++
3. The sum of the probability of an event occurring and it not occurring
is 1. pr(A) + pr(not A) = 1 or pr(not A) = 1 - pr(A)
Example: A bag contains 5 red marbles, 3 blue marbles, and 2 green marbles.
pr (red) + pr(not red) = 1
3
+ pr(not red) = 1 pr(not red) =
10
7
10

4. If two events A and B are independent (this means that the
occurrence of A has no impact at all on whether B occurs and vice versa), then
the probability of A and B occurring is the product of their individual
probabilities. pr (A and B) = pr(A) · pr(B)
Example: roll a die and flip a coin. pr(heads and roll a 3) = pr(H) and pr(3)
1 1 1
2 6 12
5. If two events A and B are mutually exclusive (meaning A cannot
occur at the same time as B occurs), then the probability of either A or B
occurring is the sum of their individual probabilities. Pr(A or B) = pr(A) + pr(B)
Example: A bag contains 5 red marbles, 3 blue marbles, and 2 green marbles.
pr(red or green) = pr(red) + pr(green)
5 2 7
10 10 10
6. If two events A and B are not mutually exclusive (meaning it is possible that
A and B occur at the same time), then the probability of either A or B occurring
is the sum of their individual probabilities minus the probability of both A and B
occurring. Pr(A or B) = pr(A) + pr(B) – pr(A and B)

13
Example: There are 20 people in the room: 12 girls (5 with blond hair
and 7 with brown hair) and 8 boys (4 with blond hair and 4 with brown hair).
There are a total of 9 blonds and 11 with brown hair. One person from the
group is chosen randomly. pr(girl or blond) = pr(girl) + pr(blond) – pr(girl and
blond) 12 9 5 16
20 20 20 20
7. The probability of at least one event occurring out of multiple events is
equal to one minus the probability of none of the events occurring. pr(at least
one) = 1 – pr(none)
Example: roll a coin 4 times. What is the probability of getting at least
one head on the 4 rolls.
pr(at least one H) = 1 – pr(no H) = 1 – pr (TTTT) = 1
1 1 1 1
2 2 2 2
= 1
1 15
16 16
8. If event B is a subset of event A, then the probability of B is less than
or equal to the probability of A. pr(B) ≤ pr(A)
Example: There are 20 people in the room: 12 girls (5 with blond hair and 7 with
brown hair) and 8 boys (4 with blond hair and 4 with brown hair). pr (girl with
brown hair) ≤ pr(girl) 7 12
20 20
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Statistics: Probability

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