Probability: Sample Space, Probability Measure, and Axioms

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Presentation transcript:

Probability: Sample Space, Probability Measure, and Axioms Jacek Wallusch _________________________________ Mathematical Statistics for International Business Lecture 2: Probability: Sample Space, Probability Measure, and Axioms

Sample Space ____________________________________________________________________________________________ definition Sample Space: a non-empty set W of all possible outcomes of an experiment Examples: demand for cars; FCOJ market; monetary policy; Mathematical Statistics: 2 infamous examples: tossing a coin, rolling a die

Getting Started ____________________________________________________________________________________________ set theory Venn diagrams: union of events set A set C set B sample space Mathematical Statistics: 2 example: even numbers, numbers divisible by 3 Why is sample space so important?

the number is even and divisible by 3 Getting Started ____________________________________________________________________________________________ set theory Union of events: example: rolling a die the number is even the number is even and divisible by 3 Mathematical Statistics: 2 example: even numbers, numbers divisible by 3 Why is the sample space so important?

intersection of events Getting Started ____________________________________________________________________________________________ set theory Venn diagrams: intersection of events set A set C set B sample space Mathematical Statistics: 2

the number is even and divisible by 3 Getting Started ____________________________________________________________________________________________ set theory Intersection of events: example: rolling a die the number is even the number is even and divisible by 3 Mathematical Statistics: 2 example: even numbers, numbers divisible by 3 Why is sample space so important?

Getting Started ____________________________________________________________________________________________ set theory Helpful properties: Probability distribution and uncertainty and risk – this topic will be reconsidered soon Mathematical Statistics: 2

compliment of A in sample space Getting Started ____________________________________________________________________________________________ set theory Complementation: set A compliment of A in sample space sample space Mathematical Statistics: 2

Getting Started ____________________________________________________________________________________________ set theory Inclusion: set A set D sample space Mathematical Statistics: 2 A is a subset of D, or conversely, D is a superset of A

Getting Started ____________________________________________________________________________________________ set theory Mutually exclusive events: no common outcomes set A set B sample space Mathematical Statistics: 2

Venn Diagrams ____________________________________________________________________________________________ example Venn diagrams: Sample Space 13 4 2 4 5 4 1 Set A Set B Set C Set D 4 2 5 1 13 correct it! Mathematical Statistics: 2

Set Theory ____________________________________________________________________________________________ Exercise 1 Events: Wernham Hogg Co., Slough Branch Payroll Sales Reps Senior Sales Reps White Collars Assistant to Regional Manager Receptionist Forklift Operators Regional Manager Other Blue Collars Mathematical Statistics: 2

Venn Diagrams ____________________________________________________________________________________________ example Wernham Hogg: White Collars Senior Sales Reps Assistant to Regional Manager Sales Reps correct it! Mathematical Statistics: 2

Algebra ____________________________________________________________________________________________ definition Algebra: a collection G of subsets of W Properties: Mathematical Statistics: 2 W – sample space

s-Algebra ____________________________________________________________________________________________ definition s-Algebra: a collection F of subsets of W Properties: Mathematical Statistics: 2 W – sample space; F – an algebra closed under a countable union

s-Algebra ____________________________________________________________________________________________ intuition What do we need s-algebra for: [1] To define probability and to assign probabilities to specific events [2] To define questions one can ask having the available data Mathematical Statistics: 2 See Dilbert (2010) on dangerously misleading results and chocking to death on lunches

s-Algebra ____________________________________________________________________________________________ Exercise 2 Problem: Valentine and Winthorpe are active players in the Frozen Concentrated Orange Juice market. Construct a s-algebra for their actions. What do they do: They sell on the high and buy on the chaep (One possible) Solution: Mathematical Statistics: 2 power set: set of all subsets of any set (including the empty set)

Probability Measure ____________________________________________________________________________________________ definition Measurable Space: a pair (W , F) Events: elements of F Probability Measure: a measure on a measurable space (W , F) satisfying: Mathematical Statistics: 2 Composite (compound) vs. elementary (simple) events FCOJ

Probability ____________________________________________________________________________________________ definition Probability Space: a triplet (W , F, P) Probability: a function P defined on F and satisfying 3 axioms: Mathematical Statistics: 2 P – probability measure

relative frequency approach Probability ____________________________________________________________________________________________ proportion Long Run Proportion: relative frequency approach problem: probability that the net-net price will not be smaller than 50% of the list price solution: step 1: gather the data on list prices and net-net for a certain period of time (e.g. one year) step 2: divide the number of all net-net prices smaller than the 50% of the list price by the total number of obsrvations Mathematical Statistics: 2 Frequentionist approach to probability

Proportion and Probability ____________________________________________________________________________________________ Exercise 3A Problem: Recognition of company Question: Schneider Electric, leading global company in energy managment and automation Probability: Prior to September 2017, how many students have ever heard of =S= Mathematical Statistics: 2

probability that customers like either Pepsi or Coca Cola Set Theory in Practice ____________________________________________________________________________________________ Exercise 4 Addition Law: problem: probability that customers like either Pepsi or Coca Cola Sample space Mathematical Statistics: 2

choosing C1 is two times more likely than choosing C2, Set Theory in Practice ____________________________________________________________________________________________ Exercise 5 Problem: A panel builder combines two elements, E1 and E2, to build a panel. Elements E1 and E2 are supplied by two companies, C1 and C2. Since C1 has a larger market share (50% vs. 25%), but products supplied by C2 are cheaper, it’s possible that a panel builder will combine E1 and E2 produced by different companies. Assuming that choosing C1 is two times more likely than choosing C2, what is the probability that a panel builder will choose at least one element supplied by C1? Mathematical Statistics: 2

Set Theory in Practice ____________________________________________________________________________________________ Exercise 5 Sample Space W: [1] event e1: E1(C1) and E2(C1); [2] event e2: E1(C1) and E2(C2); [3] event e3: E1(C2) and E2(C1); [4] event e4: E1(C2) and E2(C2) Assumptions regarding probability: Choosing C1 is two times more likely than choosing C2 Mathematical Statistics: 2

Choosing at least one component supplied by C1: event Q Set Theory in Practice ____________________________________________________________________________________________ Exercise 5 Solution: Why 2/3 and 1/3? Choosing at least one component supplied by C1: event Q Applying set theory: Mathematical Statistics: 2

problem: early morning tea or coffee? Set Theory in Practice ____________________________________________________________________________________________ Exercise 6 Mutually Exclusive Events: complimentation problem: early morning tea or coffee? Mathematical Statistics: 2

k objects are to be selected from a set of p objects Counting rules ____________________________________________________________________________________________ combinations without repetitions How to count the number of experimental combinations? k objects are to be selected from a set of p objects Probability distribution and uncertainty and risk – this topic will be reconsidered soon Factorial, binomial coefficient Mathematical Statistics: 2 hit the jackpot in Polish Lotto: pick correctly 6 numbers from 1 to 49 (13 983 816)

k objects are to be selected from a set of p objects with repetitions Counting rules ____________________________________________________________________________________________ combinations with repetitions How to count the number of experimental combinations? k objects are to be selected from a set of p objects with repetitions Probability distribution and uncertainty and risk – this topic will be reconsidered soon Mathematical Statistics: 2