Definition A random variable X is said to have a Weibull distribution with parameters α and β (α > 0, β > 0) if the pdf of X is

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1 Weibull Distribution Definition A random variable X is said to have a Weibull distribution with parameters α and β (α > 0, β > 0) if the pdf of X is { α β x α 1 e f (x; α, β) = α (x/β)α x 0 0 x < 0 Remark: 1. The family of Weibull distributions was introduced by the Swedish physicist Waloddi Weibull in We use X WEB(α, β) to denote that the rv X has a Weibull distribution with parameters α and β. Liang Zhang (UofU) Applied Statistics I July 1, / 15

2 Weibull Distribution Remark: 3. When α = 1, the pdf becomes { 1 β f (x; β) = e x/β x 0 0 x < 0 which is the pdf for an exponential distribution with parameter λ = 1 β. Thus we see that the exponential distribution is a special case of both the gamma and Weibull distributions. 4. There are gamma distributions that are not Weibull distributios and vice versa, so one family is not a subset of the other. Liang Zhang (UofU) Applied Statistics I July 1, / 15

3 Weibull Distribution Liang Zhang (UofU) Applied Statistics I July 1, / 15

4 Weibull Distribution Liang Zhang (UofU) Applied Statistics I July 1, / 15

5 Weibull Distribution Proposition Let X be a random variable such that X WEI(α, β). Then ( E(X ) = βγ ) α ( and V (X ) = β {Γ ) [ ( Γ )] } 2 α α The cdf of X is F (x; α, β) = { 1 e (x/β)α x 0 0 x < 0 Liang Zhang (UofU) Applied Statistics I July 1, / 15

6 Weibull Distribution Example: The shear strength (in pounds) of a spot weld is a Weibull distributed random variable, X WEB(400, 2/3). a. Find P(X > 410). b. Find P(X > 410 X > 390). c. Find E(X ) and V (X ). d. Find the 95th percentile. Liang Zhang (UofU) Applied Statistics I July 1, / 15

7 Weibull Distribution In practical situations, γ = min(x ) > 0 and X γ has a Weibull distribution. Example (Problem 74): Let X = the time (in 10 1 weeks) from shipment of a defective product until the customer returns the product. Suppose that the minimum return time is γ = 3.5 and that the excess X 3.5 over the minimum has a Weibull distribution with parameters α = 2 and β = 1.5. a. What is the cdf of X? b. What are the expected return time and variance of return time? c. Compute P(X > 5). d. Compute P(5 X 8). Liang Zhang (UofU) Applied Statistics I July 1, / 15

8 Lognormal Distribution Definition A nonnegative rv X is said to have a lognormal distribution if the rv Y = ln(x ) has a normal distribution. The resulting pdf of a lognormal rv when ln(x ) is normally distributed with parameters µ and σ is { 1 f (x; µ, σ) = 2πσx e [ln(x) µ]2 /(2σ 2 ) x 0 0 x < 0 Remark: 1. We use X LOGN(µ, σ 2 ) to denote that rv X have a lognormal distribution with parameters µ and σ. 2. Notice here that the parameter µ is not the mean and σ 2 is not the variance, i.e. µ E(X ) and σ 2 V (X ) Liang Zhang (UofU) Applied Statistics I July 1, / 15

9 Lognormal Distribution Liang Zhang (UofU) Applied Statistics I July 1, / 15

10 Lognormal Distribution Proposition If X LOGN(µ, σ 2 ), then E(X ) = e µ+σ2 /2 and V (X ) = e 2µ+σ2 (e σ2 1) The cdf of X is F (x; µ, σ) = P(X x) = P[ln(X ) ln(x)] ( = P Z ln(x) µ ) ( ) ln(x) µ = Φ σ σ x 0 where Φ(z) is the cdf of the standard normal rv Z. Liang Zhang (UofU) Applied Statistics I July 1, / 15

11 Lognormal Distribution Example (Problem 115) Let I i be the input current to a transistor and I 0 be the output current. Then the current gain is proportional to ln(i 0 /I i ). Suppose the constant of proportionality is 1 (which amounts to choosing a particular unit of measurement), so that current gain = X = ln(i 0 /I i ). Assume X is normally distributed with µ = 1 and σ = a. What is the probability that the output current is more than twice the input current? b. What are the expected value and variance of the ratio of output to input current? c. What value r is such that only 5% chance we will have the ratio of output to input current exceed r? Liang Zhang (UofU) Applied Statistics I July 1, / 15

12 Beta Distribution Definition A random variable X is said to have a beta distribution with parameters α, β(both positive), A, and B if the pdf of X is ( α 1 ( β 1 1 f (x; α, β, A, B) = B A Γ(α+β) Γ(α) Γ(β) x A B A) B x B A) A x B 0 otherwise The case A = 0, B = 1 gives the standard beta distribution. Remark: We use X BETA(α, β, A, B) to denote that rv X has a beta distribution with parameters α, β, A, and B. Liang Zhang (UofU) Applied Statistics I July 1, / 15

13 Beta Distribution Proposition If X BETA(α, β, A, B), then E(X ) = A + (B A) α α + β and V (X ) = (B A) 2 αβ (α + β) 2 (α + β + 1) Liang Zhang (UofU) Applied Statistics I July 1, / 15

14 Beta Distribution Liang Zhang (UofU) Applied Statistics I July 1, / 15

15 Beta Distribution Example (Problem 127) An individual s credit score is a number calculated based on that person s credit history which helps a lender determine how much he/she should be loaned or what credit limit should be established for a credit card. An article in the Los Angeles Times gave data which suggested that a beta distribution with parameters A = 150, B = 850, α = 8, β = 2 would provide a reasonable approximation to the distribution of American credit scores. [Note: credit scores are integer-valued]. a. Let X represent a randomly selected American credit score. What are the mean value and standard deviation of this random variable? What is the probability that X is within 1 standard deviation of its mean value? b. What is the approximate probability that a randomly selected score will exceed 750 (which lenders consider a very good score)? Liang Zhang (UofU) Applied Statistics I July 1, / 15