Basic Probability Distributions in Business Statistics

1. 2003 Prentice-Hall, Inc. Chap 5-1 Basic Business Statistics (9 th Edition) Chapter 5 Some Important Discrete Probability Distributions

2. 2003 Prentice-Hall, Inc. Chap 5-2 Chapter Topics The Probability of a Discrete Random Variable Covariance and Its Applications in Finance Binomial Distribution Poisson Distribution Hypergeometric Distribution

3. 2003 Prentice-Hall, Inc. Chap 5-3 Random Variable Random Variable Outcomes of an experiment expressed numerically E.g., Toss a die twice; count the number of times the number 4 appears (0, 1 or 2 times) E.g., Toss a coin; assign $10 to head and -$30 to a tail = $10 T = -$30

4. 2003 Prentice-Hall, Inc. Chap 5-4 Discrete Random Variable Discrete Random Variable Obtained by counting (0, 1, 2, 3, etc.) Usually a finite number of different values E.g., Toss a coin 5 times; count the number of tails (0, 1, 2, 3, 4, or 5 times)

5. 2003 Prentice-Hall, Inc. Chap 5-5 Probability Distribution Values Probability 0 1/4 = .25 1 2/4 = .50 2 1/4 = .25 Discrete Probability Distribution Example Event: Toss 2 Coins Count # Tails T T T T This is using the A Priori Classical Probability approach.

6. 2003 Prentice-Hall, Inc. Chap 5-6 Discrete Probability Distribution List of All Possible [ X j , P(X j ) ] Pairs X j = Value of random variable P(X j ) = Probability associated with value Mutually Exclusive (Nothing in Common) Collective Exhaustive (Nothing Left Out)

7. 2003 Prentice-Hall, Inc. Chap 5-7 Summary Measures Expected Value (The Mean) Weighted average of the probability distribution E.g., Toss 2 coins, count the number of tails, compute expected value:

8. 2003 Prentice-Hall, Inc. Chap 5-8 Summary Measures Variance Weighted average squared deviation about the mean E.g., Toss 2 coins, count number of tails, compute variance: (continued)

9. 2003 Prentice-Hall, Inc. Chap 5-9 Covariance and Its Application

10. 2003 Prentice-Hall, Inc. Chap 5-10 Computing the Mean for Investment Returns Return per $1,000 for two types of investments P( X i ) P( Y i ) Economic Condition Dow Jones Fund X Growth Stock Y .2 .2 Recession -$100 -$200 .5 .5 Stable Economy + 100 + 50 .3 .3 Expanding Economy + 250 + 350 Investment

11. 2003 Prentice-Hall, Inc. Chap 5-11 Computing the Variance for Investment Returns P( X i ) P(Y i ) Economic Condition Dow Jones Fund X Growth Stock Y .2 .2 Recession -$100 -$200 .5 .5 Stable Economy + 100 + 50 .3 .3 Expanding Economy + 250 + 350 Investment

12. 2003 Prentice-Hall, Inc. Chap 5-12 Computing the Covariance for Investment Returns P( X i Y i ) Economic Condition Dow Jones Fund X Growth Stock Y .2 Recession -$100 -$200 .5 Stable Economy + 100 + 50 .3 Expanding Economy + 250 + 350 Investment The covariance of 23,000 indicates that the two investments are positively related and will vary together in the same direction.

13. 2003 Prentice-Hall, Inc. Chap 5-13 Computing the Coefficient of Variation for Investment Returns Investment X appears to have a lower risk (variation) per unit of average payoff (return) than investment Y Investment X appears to have a higher average payoff (return) per unit of variation (risk) than investment Y

14. 2003 Prentice-Hall, Inc. Chap 5-14 Sum of Two Random Variables The expected value of the sum is equal to the sum of the expected values The variance of the sum is equal to the sum of the variances plus twice the covariance The standard deviation is the square root of the variance

15. 2003 Prentice-Hall, Inc. Chap 5-15 Portfolio Expected Return and Risk The portfolio expected return for a two-asset investment is equal to the weighted average of the two assets Portfolio risk

16. 2003 Prentice-Hall, Inc. Chap 5-16 Computing the Expected Return and Risk of the Portfolio Investment P( X i Y i ) Economic Condition Dow Jones Fund X Growth Stock Y .2 Recession -$100 -$200 .5 Stable Economy + 100 + 50 .3 Expanding Economy + 250 + 350 Investment Suppose a portfolio consists of an equal investment in each of X and Y :

17. 2003 Prentice-Hall, Inc. Chap 5-17 Doing It in PHStat PHStat | Decision Making | Covariance and Portfolio Analysis Fill in the Number of Outcomes: Check the Portfolio Management Analysis box Fill in the probabilities and outcomes for investment X and Y Manually compute the CV using the formula in the previous slide Here is the Excel spreadsheet that contains the results of the previous investment example:

18. 2003 Prentice-Hall, Inc. Chap 5-18 Important Discrete Probability Distributions Discrete Probability Distributions Binomial Hypergeometric Poisson

19. 2003 Prentice-Hall, Inc. Chap 5-19 Binomial Probability Distribution n Identical Trials E.g., 15 tosses of a coin; 10 light bulbs taken from a warehouse 2 Mutually Exclusive Outcomes on Each Trial E.g., Heads or tails in each toss of a coin; defective or not defective light bulb Trials are Independent The outcome of one trial does not affect the outcome of the other

20. 2003 Prentice-Hall, Inc. Chap 5-20 Binomial Probability Distribution Constant Probability for Each Trial E.g., Probability of getting a tail is the same each time we toss the coin 2 Sampling Methods Infinite population without replacement Finite population with replacement (continued)

21. 2003 Prentice-Hall, Inc. Chap 5-21 Binomial Probability Distribution Function Tails in 2 Tosses of Coin X P(X) 0 1/4 = .25 1 2/4 = .50 2 1/4 = .25

22. 2003 Prentice-Hall, Inc. Chap 5-22 Binomial Distribution Characteristics Mean E.g., Variance and Standard Deviation E.g., n = 5 p = 0.1 0 .2 .4 .6 0 1 2 3 4 5 X P(X)

23. 2003 Prentice-Hall, Inc. Chap 5-23 Binomial Distribution in PHStat PHStat | Probability & Prob. Distributions | Binomial Example in Excel Spreadsheet

24. 2003 Prentice-Hall, Inc. Chap 5-24 Example: Binomial Distribution A mid-term exam has 30 multiple choice questions, each with 5 possible answers. What is the probability of randomly guessing the answer for each question and passing the exam (i.e., having guessed at least 18 questions correctly)? Are the assumptions for the binomial distribution met? Yes, the assumptions are met. Using results from PHStat:

25. 2003 Prentice-Hall, Inc. Chap 5-25 Poisson Distribution Simon Poisson

26. 2003 Prentice-Hall, Inc. Chap 5-26 Poisson Distribution Discrete events (successes) occurring in a given area of opportunity (interval) Interval can be time, length, surface area, etc. The probability of a success in a given interval is the same for all the intervals The number of successes in one interval is independent of the number of successes in other intervals The probability of two or more successes occurring in an interval approaches zero as the interval becomes smaller E.g., # customers arriving in 15 minutes E.g., # defects per case of light bulbs

27. 2003 Prentice-Hall, Inc. Chap 5-27 Poisson Probability Distribution Function E.g., Find the probability of 4 customers arriving in 3 minutes when the mean is 3.6.

28. 2003 Prentice-Hall, Inc. Chap 5-28 Poisson Distribution in PHStat PHStat | Probability & Prob. Distributions | Poisson Example in Excel Spreadsheet P X x x x ( | ! e -

29. 2003 Prentice-Hall, Inc. Chap 5-29 Poisson Distribution Characteristics Mean Standard Deviation and Variance = 0.5 = 6 0 .2 .4 .6 0 1 2 3 4 5 X P( X ) 0 .2 .4 .6 0 2 4 6 8 10 X P( X )

30. 2003 Prentice-Hall, Inc. Chap 5-30 Hypergeometric Distribution n Trials in a Sample Taken from a Finite Population of Size N Sample Taken Without Replacement Trials are Dependent Concerned with Finding the Probability of X Successes in the Sample Where There are A Successes in the Population

31. 2003 Prentice-Hall, Inc. Chap 5-31 Hypergeometric Distribution Function E.g., 3 Light bulbs were selected from 10. Of the 10, there were 4 defective. What is the probability that 2 of the 3 selected are defective?

32. 2003 Prentice-Hall, Inc. Chap 5-32 Hypergeometric Distribution Characteristics Mean Variance and Standard Deviation Finite Population Correction Factor

33. 2003 Prentice-Hall, Inc. Chap 5-33 Hypergeometric Distribution in PHStat PHStat | Probability & Prob. Distributions | Hypergeometric Example in Excel Spreadsheet

34. 2003 Prentice-Hall, Inc. Chap 5-34 Chapter Summary Addressed the Probability of a Discrete Random Variable Defined Covariance and Discussed Its Application in Finance Discussed Binomial Distribution Addressed Poisson Distribution Discussed Hypergeometric Distribution