Statistics Lecture 5

Last class: measures of spread and box-plots Last Day - Began Chapter 2 on probability. Section 2.1 These Notes – more Chapter 2…Section 2.2 and 2.3 Assignment 2: 2.8, 2.12, 2.18, 2.24, 2.30, 2.36, 2.40 Due: Friday, January 27 Suggested problems: 2.26, 2.28, 2.39

Probability Probability of an event is the long-term proportion of times the event would occur if the experiment is repeated many times Read page on Interpreting probability

Probability Probability of event, A is denoted P(A) Axioms of Probability: For any event, A, P(S) = 1 If A 1, A 2, …, A k are mutually exclusive events, These imply that

Discrete Uniform Distribution Sample space has k possible outcomes S={e 1,e 2,…,e k } Each outcome is equally likely P(e i )= If A is a collection of distinct outcomes from S, P(A)=

Example A coin is tossed 1 time S= Probability of observing a heads or tails is

Example A coin is tossed 2 times S= What is the probability of getting either two heads or two tails? What is the probability of getting either one heads or two heads?

Example Inherited characteristics are transmitted from one generation to the next by genes Genes occur in pairs and offspring receive one from each parent Experiment was conducted to verify this idea Pure red flower crossed with a pure white flower gives Two of these hybrids are crossed. Outcomes: Probability of each outcome

Note Sometimes, not all outcomes are equally likely (e.g., fixed die) Recall, probability of an event is long-term proportion of times the event occurs when the experiment is performed repeatedly NOTE: Probability refers to experiments or processes, not individuals

Probability Rules Have looked at computing probability for events How to compute probability for multiple events? Example: 65% of SFU Business School Professors read the Wall Street Journal, 55% read the Vancouver Sun and 45% read both. A randomly selected Professor is asked what newspaper they read. What is the probability the Professor reads one of the 2 papers?

Addition Rules: If two events are mutually exclusive: Complement Rule

Example: 65% of SFU Business School Professors read the Wall Street Journal, 55% read the Vancouver Sun and 45% read both. A randomly selected Professor is asked what newspaper they read. What is the probability the Professor reads one of the 2 papers?

Counting and Combinatorics In the equally likely case, computing probabilities involves counting the number of outcomes in an event This can be hard…really Combinatorics is a branch of mathematics which develops efficient counting methods These methods are often useful for computing probabilites

Combinatorics Consider the rhyme As I was going to St. Ives I met a man with seven wives Every wife had seven sacks Every sack had seven cats Every cat had seven kits Kits, cats, sacks and wives How many were going to St. Ives? Answer:

Example In three tosses of a coin, how many outcomes are there?

Product Rule Let an experiment E be comprised of smaller experiments E 1,E 2, …, E k, where E i has n i outcomes The number of outcome sequences in E is (n 1 n 2 n 3 …n k ) Example (St. Ives re-visited)

Example In a certain state, automobile license plates list three letters (A-Z) followed by four digits (0-9) How many possible license plates are there?

Tree Diagram Can help visualize the possible outcomes Constructed by listing the posbilites for E 1 and connecting these separately to each possiblility for E 2, and so on

Example In three tosses of a coin, how many outcomes are there?

Example - Permuatation Suppose have a standard deck of 52 playing cards (4 suits, with 13 cards per suit) Suppose you are going to draw 5 cards, one at a time, with replacement (with replacement means you look at the card and put it back in the deck) How many sequences can we observe

Permutations In previous examples, the sample space for E i does not depend on the outcome from the previous step or sub-experiment The multiplication principle applies only if the number of outcomes for E i is the same for each outcome of E i-1 That is, the outcomes might change change depending on the previous step, but the number of outcomes remains the same

Permutations When selecting object, one at a time, from a group of N objects, the number of possible sequences is: The is called the number of permutations of n things taken k at a time Sometimes denoted P k,n Can be viewed as number of ways to select k things from n objects where the order matters

Permutations The number of ordered sequences of k objects taken from a set of n distinct objects (I.e., number of permutations) is: P k,n =n(n-1) … (n-k+1)

Example Suppose have a standard deck of 52 playing cards (4 suits, with 13 cards per suit) Suppose you are going to draw 5 cards, one at a time, without replacement How many permutations can we observe

Combinations If one is not concerned with the order in which things occur, then a set of k objects from a set with n objects is called a combination Example Suppose have 6 people, 3 of whom are to be selected at random for a committee The order in which they are selected is not important How many distinct committees are there?

Combinations The number of distinct combinations of k objects selected from n objects is: “n choose k” Note: n!=n(n-1)(n-2)…1 Note: 0!=1 Can be viewed as number of ways to select mthings taken k at a time where the order does not matter

Combinations Example Suppose have 6 people, 3 of whom are to be selected at random for a committee The order in which they are selected is not important How many distinct committees are there?

Example A committee of size three is to be selected from a group of 4 Conservatives, 3 Liberals and 2 NDPs How many committees have a member from each group? What is the probability that there is a member from each group on the committee?