# Probabilities. Probability of a event. From Random Variables to Events. From Random Variables to Events. Probability Theory I

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1 Victor Adamchi Danny Sleator Great Theoretical Ideas In Computer Science Probability Theory I CS 5-25 Spring 200 Lecture Feb. 6, 200 Carnegie Mellon University We will consider chance experiments with a finite number of possible outcomes w, w 2,..., w n The sample space of the experiment is the set of all possible outcomes. The roll of a die: = {,2,3,4,5,6} ach subset of a sample space is defined to be an event. The event: = {2,4,6} Probability of a event Let X be a random variable which denotes the value of the outcome of a certain experiment. Probabilities For any subset of, we define the probability of to be the number P() given by We will assign probabilities to the possible outcomes of an experiment. We do this by assigning to each outcome w j a nonnegative number j ) in such a way that ) + + n )= event P() w ) distribution function The function j ) is called the distribution function of the random variable X. From Random Variables to vents For any random variable X and value a, we can define the event that X = a From Random Variables to vents It s a function on the sample space ->[0, ) P() = P(X=a) = P({t X(t)=a}) It s a variable with a probability distribution on its values You should be comfortable with both views

2 xample Consider an experiment in which a coin is tossed twice. = {HH,TT,HT,TH} m(hh)=m(tt)=m(ht)=m(th)=/4 Let ={HH,HT,TH} be the event that at least one head comes up. Then probability of is P() = m(hh)+m(ht)+m(th) =3/4 xample 2 Three people, A, B, and C, are running for the same office, and we assume that one and only one of them wins. Suppose that A and B have the same chance of winning, but that C has only /2 the chance of A or B. Let be the event that either A or C wins. P() = m(a)+m(c) = 2/5+/5=3/5 Notice that it is an immediate consequence P({w}) = ) Theorem The probabilities satisfy the following properties: P(A B) = P(A) + P(B), for disjoint A and B P( ) = Proof. P() 0 P(A B) = P(A) + P(B), for disjoint A and B P(A B) w A ) B P(A) = - P(A) ) ) P(A) P(B) w A w B For any events A and B More Theorems P(A) = P(A B) + P(A B) Uniform Distribution When a coin is tossed and the die is rolled, we assign an equal probability to each outcome. For any events A and B P(A B) = P(A) + P(B) - P(A B) The uniform distribution on a sample space containing n elements is the function m defined by ) = /n for every w 2

3 xample xample Consider the experiment that consists of rolling a pair of dice. What is the probability of getting a sum of 7 or a sum of? S = { (,), (,2), (,3), (,4), (,5), (,6), (2,), (2,2), (2,3), (2,4), (2,5), (2,6), (3,), (3,2), (3,3), (3,4), (3,5), (3,6), (4,), (4,2), (4,3), (4,4), (4,5), (4,6), (5,), (5,2), (5,3), (5,4), (5,5), (5,6), (6,), (6,2), (6,3), (6,4), (6,5), (6,6) } F We assume that each of 36 outcomes is equally liely. P()= 6 * /36 P(F)= 2 * /36 P( F)= 8/36 A fair coin is tossed 00 times in a row What is the probability that we get exactly half heads? The sample space is the set of all outcomes (sequences) {H,T} 00 ach sequence in is equally liely, and hence has probability / =/2 00 = all sequences of 00 tosses Visually t = HHTTT TH Set of all 2 00 sequences {H,T} 00 vent = Set of sequences with 50 H s and 50 T s P(t) = / S Probability of event = proportion of in S / 200 3

4 Birthday Paradox How many people do we need to have in a room to mae it a favorable bet (probability of success greater than /2) that two people in the room will have the same birthday? And The Same Methods Again! Sample space = 365 x We must find sequences that have no duplication of birthdays. vent = { w two numbers not the same } P() (365 x 365 x ) Birthday Paradox Number of People Probability Infinite Sample Spaces A coin is tossed until the first time that a head turns up. = {, 2, 3, 4, } A distribution function: m(n) = 2 -n P w ) Infinite Sample Spaces Let be the event that the first time a head turns up is after an even number of tosses. = {2, 4, 6, } Conditional Probability Consider our voting example: three candidates A, B, and C are running for office. We decided that A and B have an equal chance of winning and C is only /2 as liely to win as A. P() Suppose that before the election is held, A drops out of the race. What are new probabilities to the events B and C P(B A)=2/3 P(C A)=/3 4

5 Conditional Probability Let = {w, w 2,..., w r } be the original sample space with distribution function ). Suppose we learn that the event has occurred. We want to assign a new distribution function ) to reflect this fact. It is reasonable to assume that the probabilities for w in should have the same relative magnitudes that they had before we learned that had occurred: ) = c ), where c is some constant. Also, if w is not in, we want ) = 0 By the probability law: From here, Conditional Probability ) c ) ) ) c P() ) P() ) F Conditional Probability The probability of event F given event is written P(F ) and is defined by P(F ) F ) F ) P() proportion of F to P(F ) P() Suppose we roll a white die and blac die What is the probability that the white is given that the total is 7? event A = {white die = } event B = {total = 7} S = { (,), (,2), (,3), (,4), (,5), (,6), (2,), (2,2), (2,3), (2,4), (2,5), (2,6), (3,), (3,2), (3,3), (3,4), (3,5), (3,6), (4,), (4,2), (4,3), (4,4), (4,5), (4,6), (5,), (5,2), (5,3), (5,4), (5,5), (5,6), (6,), (6,2), (6,3), (6,4), (6,5), (6,6) } Independence! A and B are independent events if P(A B ) = P(A ) P(A B ) = P(A) P(B ) P(A B ) = P ( A B ) A B = = P(B) B 6 P(B A ) = P(B ) event A = {white die = } event B = {total = 7} 5

6 Silver and Gold One bag has two silver coins, another has two gold coins, and the third has one of each One bag is selected at random. One coin from it is selected at random. It turns out to be gold What is the probability that the other coin is gold? Let G be the event that the first coin is gold P(G ) = /2 Let G 2 be the event that the second coin is gold P(G 2 G ) = P(G G 2 ) / P(G ) = (/3) / (/2) = 2/3 Note: G and G 2 are not independent Monty Hall Problem The host show hides a car behind one of 3 doors at random. Behind the other two doors are goats. You select a door. Announcer opens one of others with no prize. You can decide to eep or switch. This is Tricy? We are inclined to thin: After one door is opened, others are equally liely What to do? To switch or not to switch? Monty Hall Problem Sample space = { car behind door, car behind door 2, car behind door 3 } ach has probability /3 Staying we win if we choose the correct door P( choosing correct door ) = /3 Switching we win if we choose the incorrect door P(choosing incorrect door ) = 2/3 Monty Hall Problem Let the doors be called X, Y and Z. Let Cx, Cy, Cz be the events that the car is behind door X and so on. Let Hx, Hy, Hz be the events that the host opens door X and so on. Supposing that you choose door X, the possibility that you win a car if you switch is P(Hz Cy) + P(Hy Cz) = P(Hz Cy) P(Cy) + P(Hy Cz) P(Cz) = x /3 + x /3 = 2/3 6

7 Another Paradox Tree Diagram Consider a family with two children. Given that one of the children is a boy, what is the probability that both children are boys? /3 /2 /2 girl boy /2 /2 /2 /2 girl boy girl boy /4 /4 /4 /4 Tree Diagram /2 /2 girl boy /2 /2 /2 boy girl boy /3 /3 /3 Bayes formula Suppose we have a set of disjoint events (hypotheses ) = H H 2 H We also have an event called evidence. We want to find the probabilities for the hypotheses given the evidence. Posterior probability P(H ) j P(H )P( H ) j P(H )P( H ) j Prior probability A Quic Calculation xpected Value I flip a coin 3 times. What is the expected (average) number of heads? Let X denote the number of heads which appear: X = 0,, 2 and 3. The corresponding probabilities are /8, 3/8, 3/8, and /8. (X) = (/8) 0 + (3/8) + (3/8) 2 + (/8) 3 =.5 This is a frequency interpretation of probability 7

8 Definition: xpectation The expectation, or expected value of a random variable X is written as (X) or [X], and is [X] = x x m(x) xercise Suppose that we toss a fair coin until a head first comes up, and let X represent the number of tosses which were made. What is [X]? with a sample space m(x). and a distribution function [X] 2 2 Language of Probability vents P( A B ) Independence Random Variables xpectation Here s What You Need to Know 8

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