Statistics Lecture 16
Gamma Distribution Normal pdf is symmetric and bell-shaped Not all distributions have these properties Some pdf’s give a skewed distribution One such family is the gamma family
Gamma Function For >0, the gamma function is Properties: For any >1, ( )=( -1) ( -1) For any positive integer, n, (n)=(n-1)! ( 1/2)=
Gamma PDF A continuous random variable, X, has a gamma distribution if it has pdf: Where, >0 and >0
Gamma PDF Expected Value: Variance:
Standard Gamma PDF’s Standard gamma=incomplete gamma = 1 Table A.4 gives tabled probabilities for incomplete gamma pdf’s for some ’s
Example: (Chapter 4, #57) Suppose the time spent by a randomly selected student who uses a terminal connected to a local computer facility has a mean of 20 minutes and variance 80 minutes 2 Find and ? What is the probability that the student uses the terminal less than 24 minutes?
Example: (Chapter 4, #57) Suppose that X has a gamma(4.5,1.5) distribution Find
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Exponential Distribution
Memoryless Property