1. Lecture 10 – Introduction to Probability
Topics
Events, sample space, random variables
Examples
Probability distribution function
Conditional probabilities
Exponential distribution
Poisson distribution
8/14/04
J. Bard and J. W. Barnes
Operations Research Models and Methods
Copyright All rights reserved

2. The Monte Hall Problem
There is $1,000,000 behind one door &
$0.00 behind the other two. #1 #2 #3
You “reserve” a door (say #1) but it remains closed. Then, Monte opens one of the other two doors (say #2).
The door Monte opens will be empty!
(Monte wants to keep the money.)

3. Stay with the original door we chose?
What’s the best strategy?
Stay with the original door we chose?
Switch to the other unopened door?
We are interested in probability as it relates to helping us make good or optimal decisions.

4. Another Example Coins & Drawers One drawer has 2 gold coins
One drawer has 1 gold & 1 silver coins
One drawer has 2 silver coins
You select a drawer and randomly select one coin
It turns out that your coin is gold
What is the probability that the other coin is gold ?

5. Probability Overview The sample space, S
= set of all possible outcomes of an experiment
Events are subsets of the sample space.
An event occurs if any of its elements occur when the experiment is run.
Two events A & B are mutually exclusive
if their intersection is null.
Elements of S are called realizations
outcomes, sample points, or scenarios
The choice of the sample space depends on the question that you are trying to answer.

6. Roll Two Dice Two possible sample spaces are:
S2 = { 2, 3, 4, , 11, 12 } (sum of the two values)
Examples of Events:
A1 = “the sum of the face values is 3”
Under S1 : A1 = { (1,2), (2,1) } ;
Under S2 : A1 = { 3 }
A2 = “one die has value 3 & the other value 1”
Under S1 : A2 = { (1,3), (3,1) } ;
Under S2 : not an event

7. Probability
Intuitive: The proportion of time that an event occurs when the same experiment is repeated
Mathematical: A probability measure P is defined on the set of all events satisfying the following:
P(A) ³ 0 " A  S
P(S) = P(A1 È A2 È · · · ) = 1
(3) Mutually exclusive events Ai imply that
P(A1 È A2 È · · · ) = P(A1) + P(A2 ) + · · ·
(4) P(A) = 1 - P(A) where A = complement of A
(5) P(AÈB) = P(A) + P(B) - P(AB)
(6) If A & B are independent, then P(AB) = P(A)P(B) _

8. (6) The conditional probability of event A given B is
P(A|B) =
P(AB)
P(B)
(7) If A & B are independent events then
In this case B gives “no information” as to whether A will occur
P(A | B) =
P(A
B)
P(B) =
P(A)P(B)
= P(A).

9. Probability Calculations
Example: roll a balanced dice
Sample space : S = { (H), (T) }
Event H: head appears (H)
S = H  T and H)  T = 
Event T: tail appears (T)
Intuitively: P(H) = P(T) = ½.
How to prove it mathematically?
Proof:
1 = P(S) = P ( H  T )
According to (2)
According to (3)
= P (H) + P (T)
Equal chance
P (H) = P (T) = 1/2

10. Basic Conditional Probabilities
Example: You roll 2 dice and were told that the sum is 7. What is probability that the first die is 2?
Define
Event B: the sum of the two dice is 7
Event A: the first dice is 2
The conditional probability of event A given B is
P(A|B) =
P(AB)
P(B)
B = { (1,6), (2,5), (3,4), (4,3), (5,2), (6,1) } and A  B = { (2,5) }
P(A | B) =
P (A  B )
P(B)
1/36
6/36 = 1 6

11. Computing Conditional Probabilities
You roll 2 dice and double your money if the sum is ³ 8
and lose if the sum is £ 7
However, the rolls occur in sequence and
you do not get to observe the first roll.
You are simply told either “the value is ³ 4” or “the value is £ 3”.
After being told this you get to place your bet.
A = event that you win = { (2,6), (3,5), (3,6), (4,4), (4,5), (4,6),
(5,3), (5,4), (5,5), (5,6), (6,2), (6,3), (6,4), (6,5), (6,6) }
B = you are told “³ 4” after the first roll
= { (4,1), (4,2), (4,3), (4,4), (4,5), (4,6), (5,1), (5,2), (5,3),
(5,4), (5,5), (5,6), (6,1),(6,2), (6,3), (6,4), (6,5), (6,6) }

12. 36 _ The goal is to calculate: win = P(A|B) and lose = P(A|B). P(A I
P(B)
A  B = { (4,4), (4,5), (4,6), (5,3), (5,4), (5,5), (5,6),
(6,2), (6,3), (6,4), (6,5), (6,6) }
Each realization (i,j ) is equally likely
P(A|B) =
|AB|
|B| =
12/36
18/36
= 2/3. 36
In a similar manner, we can show that _
P(A | B)= 6/18 = 1/3
However, P(A|B) = 1 – P(A|B).

13. More Conditional Probability Calculations
P(A  B)
P(A|B) =
P(B  A)
and P(B|A) =
P(B)
P(A)
P(A  B) = P(A|B)P(B) and P(B  A) = P(B|A)P(A)
This leads to Bayes’ Theorem
P(A|B)P(B) = P(B|A)P(A) or P(A|B) =
P(B|A)P(A)
P(B)

14. Example: Coins & Drawers1 2 3
Example: Coins & Drawers
Drawer 1 has 2 gold coins
Drawer 2 has 1 gold & 1 silver coin
Drawer 3 has 2 silver coins
D1 = event that we pick drawer 1
G1 = event that the first coin we select is gold
P(D1|G1) = probability that both coins are gold
given that the first is gold

15. Coin & Drawer Computations
P(Di) = 1/3, i = 1, 2, 3
P(G1) = (1)(1/3) + (1/2)(1/3) + (0)(1/3)
P(D1|G1) =
P(G1 | D1) P(D1)
P(G1) =
(1)(1/3)
(1)(1/3) + (1/2)(1/3) + (0)(1/3)
= 2/3

16. Example: The Monte Hall Problem#1 #2 #3
There is $1,000,000 behind one of the doors &
$0.00 behind the others.
Say you pick door #1 and then Monte opens one of the other two doors.
If you don’t switch, P(Win) = 1/3.
Optimal Strategy: Switch to the door that remains closed.

17. Events:
D1 = you end up choosing door 1
D2 = you end up choosing door 2
D3 = you end up choosing door 3
L = prize is behind door 1
M = prize is behind door 2
R = prize is behind door 3
Event you win: W = (D1Ç L) È (D2 Ç M) È (D3 Ç R)
These are mutually exclusive so
P(W) = P(D1Ç L) + P(D2 Ç M) + P(D3 Ç R)
= P(D1|L) P(L) + P(D2|M) P(M) + P(D3|R) P(R)
= (0) (1/3)+ (1) (1/3) + (1) (1/3) = 2/3

18. Random Variables Sample space: (1,1), …(6,6)
R.V. is a real-valued functions defined on the sample space
Example: toss two dice,
Sample space: (1,1), …(6,6)
Quantity of interest: sum of the two dices, 2, 3, … 12
RV: function that maps sample space
to the quantity of interest
Define : X  the RV, which is defined as the sum of 2 fair dice
P{X = 2} = P{ (1,1) } = 1/36
P(X = 3} = P{ (1,2), (2,1)} = 2/36
P{X = 4} = P{ (1,3), (2,2), (3,1) } = 3/36
...
P{X = 7} = P{ (1,6), (2,5), (3,4), (4,3), (5,2), (6,1) } = 6/36 …
P{X = 12} = P{ (6,6) } = 1/36

19. Classification of Random Variables
Discrete random variables:
take finite or a countable num of possible values
Probability mass function (pmf):
Probability of an outcome, p(a) = P{X = a}
Examples: Bernoulli , Binomial, Geometric and Poisson
Continuous random variables:
takes uncountable number of possible values
Probability density function (pdf):
f(a) = P{X  a } = ò- f(x)dx
Examples: Uniform, Exponential, Gamma, Normal a

20. Expected Value Continuous RV, X: E[X] = ò x f(x)dx-
Discrete random varible, X:
E[X] = xS x p(x )

21. The Exponential Distribution
This is the most frequently used distribution for modeling interarrival and service times in queueing systems. In many applications, it is regarded as doing a good job of balancing realism and mathematical tractability.
Time between customer arrivals at ATM
Time until a machine in work center breaks
Time between calls to reservation system

22. Exponential Random Variable, T
Let  be parameter of the exponential distribution.
f(t ) = {
e –t, t ³ 0
0, t < 0
probability density
function (pdf) f
Pr{T £ t) = ò t
e –u du = 1 – e –t (t ³ 0)
Expected value of T:
E[T] = ò uf(u)du = ò ue –u du = 1/
Variance of T:
Var[T] = E[T – 1/]2 = ò (u – 1/)2e –u du = 1/2

23. Memoryless Property of an Exponential Random Variable
Pr{T  a + b | T  b} = Pr{T  a} a, b  0
This says that if we’ve already waited b units of time and the event has not occurred (e.g., the next customer hasn’t arrived) then the probability distribution governing the remaining time until its occurrence is the same as it would be if the system were restarted.
That is, the process “forgets” that the event has not yet occurred. This is often a good assumption when the time of the next arrival is not “influenced” by the last arrival.

24. = e –(a+b) = e –ae –b = e –a
Proof:
P(A I B) =
P(AB)
P(B)
P(T > a + b | T > b) =
P(T > b) =
P(T > a + b and T > b)
P(T > a + b)
= e –(a+b) = e –ae –b = e –a
e –b e –b
= P(T > a)

25. Using the Exponential Distribution
Calls arrive at an emergency hotline switchboard at a rate of 3 per hour. It is now 8 AM.
(a) What is the probability that the first call arrives after 8:30 AM?
(b) What is the probability that the first call arrives between 8:15 and 8:45 AM?
(c) Given that no calls have arrived by 8:30 AM what is the probability that one or more calls arrive by 9 AM?

26. Solutions ¬ memoryless property
Let T = interarrival time random variable.
Assume T is exponentially distributed with  = 3, so
F(t) = P(T £ t) = 1 – e –3t and f(t) = 3e –3t
(a) P(T > ½) = e –3(1/2) =
(b) P(¼ < T < ¾)
= 3e
-3u
du = F(¾) – F(¼) ò
1/4
3/4
= [ 1 - e
-9/4 ]
[ 1
-3/4
= e =
(c) P(T £ 1 | T ³ ½) = 1 - P(T > 1 | T ³ ½)
= 1 - P(T > ½)
¬ memoryless property
= 1 - e –3(1/2) =

27. Relationship between Exponential Distribution
and the Poisson “Counting” Distribution
The exponential distribution is a continuous (time) distribution that, in our setting, models the waiting time until the next event (e,g., next customer arrival).
The Poisson distribution is a discrete (counting) distribution which governs the total number of events (e.g., arrivals) in the time interval (0, t ).
FACT: If inter-event times are independent exponential rv’s with rate  then the number of events that occur in the interval (0, t ) is governed by a Poisson distribution with rate . And vice-versa.

28. Poisson Distribution
Xt = # of events (e.g., arrivals) in the interval (0, t ) is governed by the following probability mass function
Pr{ Xt = n } = (t)ne –t/n! , n = 0, 1, . . .
E[ Xt ] =  t
arrival rate
length of interval
Note: Pr{ Xt = 0 } = Pr{ T > t } = e –t

29. Example (revisited)
Calls arrive at an emergency hotline switchboard once every 20 minutes on average. It is now 8:00 AM.
Use the Poisson distribution to answer the following questions.
What is the probability that the first call arrives after 8:30 AM?
Solution: Xt ~ Poisson(3t); X½ ~ Poisson(3/2), where = # of arrivals in first ½ hour.
P(X½ = 0) = (3/2)0e –3/2 = e –3/2 = 0.223

30. (b) What is the probability that the first call arrives between 8:15 AM and 8:45 AM.
Solution: Let Y1 = #of arrivals in first 15 minutes
Y2 = # of arrivals between 8:15 and 8:45 AM
We must determine: Pr{ Y1 = 0 and Y2 ³ 1 }
However, Y1 ~ Poison(3/4), Y2 ~ Poisson(3/2)
and the events {Y1 = 0} and {Y2 ³ 1} are independent.
P(Y1 = 0 and Y2 ³ 1) = P(Y1 = 0)P(Y2 ³ 1) = P(Y1 = 0)[1 - P(Y2 = 0)] =
(3/4) 0! [1 -
(3/2)
e –3/4
e –3/2 ] = e –3/4 - e –9/4 =

31. (c) Given that no calls have arrived by 8:30 AM what’s the probability that one or more calls arrive by 9:00 AM?
Solution: Pr{ Y2 ³ 1 | Y1 = 0 }, where
Y1 = #of arrivals between 8:00 and 8:30 AM
Y2 = # of arrivals between 8:30 and 9:00 AM
and
Y1 ~ Poisson(3/2), Y2 ~ Poisson(3/2)
Y1 and Y2 are independent because they are defined on non-overlapping time intervals.
= 1 -
(3/2) 0. l
e –3/2 = 1 - e –3/2 =
P(Y2 ³ 1 | Y1 = 0 ) = P(Y2 ³ 1) = 1 - P(Y2 = 0)

32. Multiple Arrival Streams
If customers of type 1 arrive according to a Poisson process with rate 1 and customers of type 2 arrive according to an independent Poisson process with rate 2 then customers arrive, regardless of type, according to a Poisson process with rate  = 1 + 2.
Generalization: Let Ti ~ exp(i), i = 1,…,n
and define T = min{ T1,…,Tn }.
Then T ~ exp(), where  = 1 + 2 + · · · + n
Restated: The minimum of several independent exponential rv’s is an exponential random variable with a rate that is the sum of the individual rates.

33. What you Should Know about Probability
How to identify events and the corresponding sample space
How to work with conditional probabilities
How to work with probability functions (e.g., normal, exponential, uniform, discrete)
How to work with the Poisson distribution