# Section 5 Part 2. Probability Distributions for Discrete Random Variables

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1 Section 5 Part 2 Probability Distributions for Discrete Random Variables

2 Review and Overview So far we ve covered the following probability and probability distribution topics Probability rules Probability tables to describe the distributions of Nominal variables Probability density curves for continuous variables particularly the Normal Distribution Section 5 Part 2 covers two probability distributions for discrete random variables with particular focus on Binomial Distribution Poisson Distribution PubH 6414 Section 5 Part 2 2

3 Review: Probability Distributions Any characteristic that can be measured or categorized is called a variable. If the variable can assume a number of different values such that any particular outcome is determined by chance it is called a random variable. Every random variable has a corresponding probability distribution. The probability distribution applies the theory of probability to describe the behavior of the random variable. PubH 6414 Section 5 Part 2 3

4 Discrete Random Variable A discrete random variable X has a finite number of possible integer values. The probability distribution of X lists the values and their probabilities in a table Value of X x 1 x 2 x 3 x k Probability p 1 p 2 p 3 p k 1. Every probability p i is a number between 0 and The sum of the probabilities must be 1. PubH 6414 Section 5 Part 2 4

5 Distribution of number of people in a family (U.S data) The probability distribution of number of people in a family is summarized in a pie chart. The sum of the probabilities 1.0 PubH 6414 Section 5 Part 2 5

6 Probability Distributions for Discrete Random Variables With a finite number of possible values the probability distributions for discrete random variables can be summarized in People in family Probability Probability Distribution Probability Number of people in family PubH 6414 Section 5 Part 2 6

7 Bernoulli trial Bernoulli trial The event (or trial) results in only one of two mutually exclusive outcomes success/failure Probability of success is known, P(success) π Examples: A single coin toss (heads or tails), P(heads) π 0.5 Survival of an individual after CABG surgery, P(survival) π 0.98 Pick an individual from the U.S. population, P(obese) π 0.31 PubH 6414 Section 5 Part 2 7

8 Binomial Distribution The binomial distribution describes the distribution of outcomes from a series of trials where each trial meets certain conditions. String together n independent Bernoulli trials. Examples: Toss a coin 10 times, X number of head, P(X 3 heads) Number of people who survive out of 1000 people after CABG surgery, X number of survivors. P(X 800 survived) A random sample of 300 from the U.S. population, X number in sample who are obese. P(X 200 are obese) PubH 6414 Section 5 Part 2 8

9 Binomial Distribution IF the following conditions are met Each trial has a binary outcome (One of the two outcomes is labeled a success ) The probability of success is known and constant over all trials The number of trials is specified The trials are independent. That is, the outcome from one trial doesn t affect the outcome of successive trials THEN The Binomial distribution describes the probability of X successes in n trials PubH 6414 Section 5 Part 2 9

10 Examples of Binomial distribution A classic example of the binomial distribution is the number of heads (X) in n coin tosses Does it meet the criteria? What is the range of X? PubH 6414 Section 5 Part 2 10

11 Practice Exercise For each coin toss the outcome is either Heads or Tails In a series of two coin tosses, what are the 4 possible combinations of outcomes? Calculate the probability of 0, 1, or 2 heads in 2 coin tosses Number of heads Probability PubH 6414 Section 5 Part 2 11

12 Parameters of Binomial Distribution The Notation for a binomial distribution is X ~ B (n, π) which is read as X is distributed binomial with n trials and probability of success in one trial equal to π PubH 6414 Section 5 Part 2 12

13 Formula for Binomial Distribution Using this formula, the probability distribution of a binomial random variable X can be calculated if n and π are known P( X ) n! π X ( 1 π ) X!( n X )! n X n! is called n factorial n(n-1)(n-2)...(1) Example: 5! 5*4*3*2*1 120 Note: 0! 1 π 0 1 PubH 6414 Section 5 Part 2 13

14 Another coin toss example What is the probability of X heads in 6 coin tosses? Success heads n 6 trials π 0.5 X number of heads in 6 tosses which can range from 0 to 6 X has a binomial distribution with n 6 and π 0.5 X ~ B (6, 0.5) PubH 6414 Section 5 Part 2 14

15 Binomial Calculation for X ~ B (6, 0.5) 0) ( 6) ( 1) ( 5) ( 2) ( 4) ( * ( )! 3!(6 6! 3) ( * ( )! 2!(6 6! 2) ( * ( )! 1!(6 6! 1) ( ( )! 0!(6 6! 0) ( ) ) ) ) X P X P X P X P X P X P X P X P X P X P PubH 6414 Section 5 Part 2 15

16 X ~ B (6, 0.5) The binomial distribution for the number of heads in 6 coin tosses is summarized in the table # Heads Probability This probability distribution is symmetric because the P(Heads) P(Tails) 0.5 PubH 6414 Section 5 Part 2 16

17 X ~ B(6, 0.5) For a small number of possible values, the binomial distribution can be plotted as a bar chart X ~B(6, 0.5) Probability Number of heads in 6 coin tosses PubH 6414 Section 5 Part 2 17

18 A clinical application If 5 year survival rate for a certain group of prostate cancer patients is known to be 0.80, what is the probability that X of 10 men with prostate cancer survive at least 5 years. 5 year survival is a binary outcome (Yes or No) Success is defined as survival for at least 5 years n number of trials 10 subjects π probability of success 0.80, assumed to be constant for all subjects X is the number of patients surviving at least 5 years. X can range from 0-10 X ~ B (10, 0.8) PubH 6414 Section 5 Part 2 18

19 Binomial Distribution for X~B(10, 0.8) Number of patients surviving 5 years Probability 0 < < The binomial distribution formula was used to calculate the probabilities that X of the 10 men would survive at least 5 years. Notice that this distribution is skewed towards the larger values. This is because the P(success) is > 0.5. PubH 6414 Section 5 Part 2 19

20 Plotting X~B(10,0.8) X ~ B(10,0.8) Probability Number of patients surviving 5 years out of 10 patients PubH 6414 Section 5 Part 2 20

21 Another clinical example In a sample of 8 patients with a heart attack, what is the probability that X will die if the probability of death from a heart attack Assume that the probability of death is the same for all patients. Death from heart attack is a binary variable (Yes or No) Success in this case is defined as death from heart attack n number of trials 8 patients π 0.03 probability of success X number of deaths. X ranges from 0 8 X ~ B (8, 0.03) PubH 6414 Section 5 Part 2 21

22 Binomial Distribution for X~B(8, 0.03) Number of deaths from heart attack Probability < < < < <0.001 The binomial distribution formula was used to calculate the probabilities that X of the 8 patents with a heart attack die Notice that this distribution is skewed towards the smaller values. This is because the P(death) is very small. PubH 6414 Section 5 Part 2 22

23 Plotting X ~ B(8, 0.03) X ~ B(8, 0.03) Probability Number of deaths from heart attack out of 8 patients PubH 6414 Section 5 Part 2 23

24 Online Binomial Calculator The binomial distribution formula involves timeconsuming calculations for larger values of n. The following website will calculate the binomial probabilities if you provide n, π, and X PubH 6414 Section 5 Part 2 24

25 Binomial Calculations in R Two functions P(Xx) use >dbinom(x, n, π) P(X x) use >pbinom(x, n, π) PubH 6414 Section 5 Part 2 25

26 Normal Approximation of the Binomial Distribution When n π > 5 and n*(1-π) > 5, the binomial distribution can be approximated by a normal distribution. Most often in health research, the normal approximation to the binomial distribution is used because the sample sizes are large enough. If not, the exact binomial formula can be used PubH 6414 Section 5 Part 2 26

27 Mean and standard deviation of binomial distribution The mean of the binomial distribution n*π The mean is the expected value of X The variance of the binomial distribution n π(1-π) The standard deviation of the binomial distribution nπ ( 1 π ) The normal approximation to the binomial. X ~ PubH 6414 Section 5 Part 2 27

28 Normal Approximation to the Binomial Distribution Example Osteopenia is a decrease in bone mineral density that can be a precursor condition to osteoporosis The risk of osteoporosis for those with osteopenia varies depending on the age of the woman. Suppose you have a group of 100 women with osteopenia. The risk of having osteoporosis diagnosed in the following year 0.20 for these women. This is an example of a binomial distribution n 100 Success osteoporosis diagnosed in following year P(success) risk 0.20 PubH 6414 Section 5 Part 2 28

29 Normal Approximation for X~B(100, 0.20) X is distributed binomial with n100 and π 0.20 Is the Normal approximation to this binomial distribution appropriate? What are the mean and SD of this normal distribution? Mean Var Std dev. PubH 6414 Section 5 Part 2 29

30 Normal Approximation to Example Use the normal approximation to the binomial to find the probability that 10 or fewer women in this group of 100 are diagnosed with osteoporosis in the next year: X ~ N(20, 4) > pnorm(10,20,4) [1] > pnorm(-2.5,0,1) [1] PubH 6414 Section 5 Part 2 30

31 Normal Approximation Example What is the probability that between 10 and 20 women in this group of 100 are diagnosed with osteoporosis in the next year? X ~ N(20, 4) > pnorm(20,20,4) - pnorm(10,20,4) [1] > pnorm(0,0,1) - pnorm(-2.5,0,1) [1] PubH 6414 Section 5 Part 2 31

32 Normal Approximation Example What is the probability that 20 or more women will be diagnosed with osteoporosis in the next year? X ~ N(20, 4) > 1-pnorm(20,20,4) [1] 0.5 PubH 6414 Section 5 Part 2 32

33 Poisson Distribution Another probability distribution for discrete variables is the Poisson distribution Named for Simeon D. Poisson, , French mathematician The Poisson distribution is used to determine the probability of the number of events occurring over a specified time or space. Examples of events over space or time: -number of cells in a specified volume of fluid -number of calls/hour to a help line -number of emergency room beds filled/ 24 hours PubH 6414 Section 5 Part 2 33

34 Poisson Distribution Like the binomial distribution and the normal distribution, there are many Poisson distributions. Each Poisson distribution is specified by the average rate at which the event occurs. The rate is notated with λ λ lambda, Greek letter L There is only one parameter for the Poisson distribution PubH 6414 Section 5 Part 2 34

35 Poisson Distribution Formula The probability that there are exactly X occurrences in the specified space or time is equal to P( X ) λ x e λ X! PubH 6414 Section 5 Part 2 35

36 What Does it Look Like? Notice that as λ increases the distribution begins to resemble a normal distribution The horizontal axis is the index X. The function is defined only at integer values of X. The connecting lines are only guides for the eye and do not indicate continuity. Source: PubH 6414 Section 5 Part 2 36

37 Normal Approximation of Poisson Distribution If λ is 10 or greater, the normal distribution is a reasonable approximation to the Poisson distribution The mean and variance for a Poisson distribution are the same and are both equal to λ The standard deviation of the Poisson distribution is the square root of λ PubH 6414 Section 5 Part 2 37

38 Poisson Distribution Example A large urban hospital has, on average, 80 emergency department admits every Monday What is the probability that there will be more than 100 emergency room admits on a Monday? λ is the rate of admits / day on Monday 80 Use the normal approximation since λ > 10 The normal approximation has mean 80 and SD 8.94 (the square root of ) PubH 6414 Section 5 Part 2 38

39 Poisson Distribution Example What is the probability that there are > 100 admits to the ER on a Monday? This information could be used in determining staffing needs for the emergency department. > 1-ppois(100,80) [1] > 1-pnorm(100,80,8.94) [1] PubH 6414 Section 5 Part 2 39

40 Overview of Probability Distributions for Discrete Data For a limited number of discrete outcomes the probability distribution can be presented in a table or chart with the probability for each outcome identified. The Binomial distribution can be approximated by a normal distribution for large enough number of trials and probability. n* π > 5 and n*(1 π) > 5 The Poisson distribution can be approximated by a normal distribution for λ > 10. PubH 6414 Section 5 Part 2 40

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