Lecture 7 Sampling Distribution Let be independent identically distributed (i.i.d.) random variables. If we want to estimate the population mean , we take sample , and use ̅ ∑ to estimate . Definition: A statistic is a function of the observable random variables in a sample and known constants. Example: ̅ ∑ All statistics have probability distributions, which we call their sampling distributions. Example: Toss a die three times. . Then ̅ Theorem: Let Note: Proof: ̅ √ . . Example: (#7.11) A forester studying the effects of fertilization on certain pine forests is interested in estimating the average basal area of pine trees. In studying basal areas, he has discovered that these measurements are normally distributed with standard deviation appr. 4 sq. inches. If the forester samples n = 9 trees, find the probability that the sample mean will be within 2 sq. in. of the population mean. Solution: Theorem: Let . Then ̅ , and ∑ √ . Proof: done. Example: (∑ Solution: . Find a number b such that ) . Suppose we want to make an inference about the population variance based on from normal population. We estimate by the sample variance: ∑ Theorem: Let Also ̅ and Proof: ̅ . Then are independent random variables. . Definition: Let independent, and √ degrees of freedom. . Then, if Z and W are is said to have a t distribution with Example: (#7.30) Let independent, then (a) Find √ and . ; (b) Given Show that Solution: . Z and W are . and ; Definition: Let and or be independent. Then (F distribution) Suppose we want to compare the variances of two normal populations. We take a sample of size from one population, and a sample of size from another. We look at Theorem: Proof: . . Example: If we take independent samples of size and from two normal populations with equal population variances, find b such that Solution: Example: (#7.34) Show Solution: ( ) . Central Limit Theorem We already showed that if is a random sample from any ̅ distribution with mean and variance , then and ̅ . Now we want to develop an approximation for the sampling distribution of ̅ (regardless of the distribution of the population). Consider random sample of size n from exponential distribution with mean 10: { Sample Average of 1000 size sample means n=5 n = 25 Variance of 1000 sample means 19.63 3.93 ̅ 9.86 9.95 10 10 Theorem (CLT): Let . Define ̅ 20 4 be i.i.d. with and ̅ ∑ √ Then √ ∫ √ for all u. In other words, ̅ is asymptotically normally distributed with mean and variance . CLT can be applied to and from any distribution provided that are both finite and n is large. Example: (#7.42) The fracture strength of tempered glass averages 14 and has standard deviation 2. (a) What is the probability that the average fracture strength of 100 randomly selected pieces of this glass exceeds 14.5? (b) Find the interval that includes, with probability 0.95, the average fracture strength of 100 randomly selected pieces of this glass. Solution: Normal Approximation to Binomial Distribution Example: (#7.80) The median age of residents of the US is 31 years, If a survey of 100 randomly selected US residents is to be taken, what is the appr. probability that at least 60 will be under 31 years of age? Solution: