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Business Statistics: A Decision-Making Approach 7th Edition Chapter Introduction to Sampling Distributions Business Statistics: A Decision-Making Approach, 7e © 2008 Prentice-Hall, Inc Chap 7-1 Chapter Goals After completing this chapter, you should be able to: Define the concept of sampling error Determine the mean and standard deviation for the sampling distribution of the sample mean, x _ Determine the mean and standard deviation for the sampling distribution of the sample proportion, p _ Describe the Central Limit Theorem and its importance Apply sampling distributions for both x and p _ _ Business Statistics: A Decision- Chap 7-2 Sampling Error Sample Statistics are used to estimate Population Parameters ex: X is an estimate of the population mean, μ Problems: Different samples provide different estimates of the population parameter Sample results have potential variability, thus sampling error exits Business Statistics: A Decision- Chap 7-3 Calculating Sampling Error Sampling Error: The difference between a value (a statistic) computed from a sample and the corresponding value (a parameter) computed from a population Example: (for the mean) Sampling Error = x - μ where: x = sample mean μ = population mean Business Statistics: A Decision- Chap 7-4 Review Population mean: Sample Mean: x ∑ μ= x ∑ x= i N i n where: μ = Population mean x = sample mean xi = Values in the population or sample N = Population size n = sample size Business Statistics: A Decision- Chap 7-5 Example If the population mean is μ = 98.6 degrees and a sample of n = temperatures yields a sample mean of x = 99.2 degrees, then the sampling error is x − μ = 99.2 − 98.6 = 0.6 degrees Business Statistics: A Decision- Chap 7-6 Sampling Errors Different samples will yield different sampling errors The sampling error may be positive or negative ( may be greater than or less than μ) x The expected sampling error decreases as the sample size increases Business Statistics: A Decision- Chap 7-7 Sampling Distribution A sampling distribution is a distribution of the possible values of a statistic for a given size sample selected from a population Business Statistics: A Decision- Chap 7-8 Developing a Sampling Distribution Assume there is a population … Population size N=4 Random variable, x, is age of individuals Values of x: 18, 20, 22, 24 (years) Business Statistics: A Decision- A B C D Chap 7-9 Developing a Sampling Distribution (continued) Summary Measures for the Population Distribution: x ∑ μ= P(x) i N 18 + 20 + 22 + 24 = = 21 σ= ∑ (x i − μ) N 2 = 2.236 Business Statistics: A Decision- 18 20 22 24 A B C D Uniform Distribution x Chap 7-10 Sampling Distribution Properties (continued) The sample mean is a consistent estimator (the value of x becomes closer to μ as n increases): Population x Small sample size As n increases, x σ x = σ/ n decreases Business Statistics: A Decision- Larger sample size μ x Chap 7-20 If the Population is not Normal We can apply the Central Limit Theorem: Even if the population is not normal, …sample means from the population will be approximately normal as long as the sample size is large enough …and the sampling distribution will have and μx = μ Business Statistics: A Decision- σ σx = n Theorem Chap 7-21 Central Limit Theorem As the sample size gets large enough… n↑ Business Statistics: A Decision- the sampling distribution becomes almost normal regardless of shape of population x Chap 7-22 If the Population is not Normal (continued) Sampling distribution properties: Population Distribution Central Tendency μx = μ Variation σ σx = n x μ Sampling Distribution (becomes normal as n increases) Larger sample size Smaller sample size (Sampling with replacement) Business Statistics: A Decision- μx x Chap 7-23 How Large is Large Enough? For most distributions, n > 30 will give a sampling distribution that is nearly normal For fairly symmetric distributions, n > 15 is sufficient For normal population distributions, the sampling distribution of the mean is always normally distributed Business Statistics: A Decision- Chap 7-24 Example Suppose a population has mean μ = and standard deviation σ = Suppose a random sample of size n = 36 is selected What is the probability that the sample mean is between 7.8 and 8.2? Business Statistics: A Decision- Chap 7-25 Example (continued) Solution: Even if the population is not normally distributed, the central limit theorem can be used (n > 30) … so the sampling distribution of normal … with mean …and standard deviation μ is approximately x =μ= x Business Statistics: A Decision- σ σx = = = 0.5 n 36 Chap 7-26 Example (continued) Solution (continued) find z-scores: μ μ 7.8 - 8.2 - x P(7.8 < μ x < 8.2) = P < < σ 36 n 36 = P(-0.4 < z < 0.4) = 0.3108 Population Distribution ??? ? ?? ? ? ? ? ? Sampling Distribution ? Sample 7.8 8.2 x μ=8 μx = Business Statistics: A Decision- Standard Normal Distribution 1554 +.1554 Standardize x -0.4 μz = 0.4 z Chap 7-27 Population Proportions, π π = the proportion of the population having some characteristic Sample proportion ( p ) provides an estimate of π : x number of successes in the sample p= = n sample size If two outcomes, p has a binomial distribution Business Statistics: A Decision- Chap 7-28 Sampling Distribution of p Approximated by a normal distribution if: P( p ) nπ ≥ n(1− π) ≥ where and μp = π Sampling Distribution p π(1− π) σp = n (where π = population proportion) Business Statistics: A Decision- Chap 7-29 z-Value for Proportions Standardize p to a z value with the formula: p−π z= = σp If sampling is without replacement and n is greater than 5% of the population size, then σ p must use the finite population correction factor: Business Statistics: A Decision- p−π π(1− π) n σp = π(1− π) N − n n N −1 Chap 7-30 Example If the true proportion of voters who support Proposition A is π = 4, what is the probability that a sample of size 200 yields a sample proportion between 40 and 45? i.e.: if π = and n = 200, what is P(.40 ≤ p ≤ 45) ? Business Statistics: A Decision- Chap 7-31 Example (continued) if π = and n = 200, what is P(.40 ≤ p ≤ 45) ? Find σ p : Convert to standard normal: π(1− π) 4(1− 4) σp = = = 03464 n 200 45 − 40 40 − 40 P(.40 ≤ p ≤ 45) = P ≤z≤ 03464 03464 = P(0 ≤ z ≤ 1.44) Business Statistics: A Decision- Chap 7-32 Example (continued) if π = and n = 200, what is P(.40 ≤ p ≤ 45) ? Use standard normal table: P(0 ≤ z ≤ 1.44) = 4251 Standardized Normal Distribution Sampling Distribution 4251 Standardize 40 45 p Business Statistics: A Decision- 1.44 z Chap 7-33 Chapter Summary Discussed sampling error Introduced sampling distributions Described the sampling distribution of the mean For normal populations Using the Central Limit Theorem Described the sampling distribution of a proportion Calculated probabilities using sampling distributions Discussed sampling from finite populations Business Statistics: A Decision- Chap 7-34 ... samples of size n is equal to the population standard deviation divided by the square root of the sample size: σ σx = n Business Statistics: A Decision- Theorem Chap 7-15 If the Population is... distribution will have and μx = μ Business Statistics: A Decision- σ σx = n Theorem Chap 7-21 Central Limit Theorem As the sample size gets large enough… n↑ Business Statistics: A Decision- the... size If two outcomes, p has a binomial distribution Business Statistics: A Decision- Chap 7-28 Sampling Distribution of p Approximated by a normal distribution if: P( p ) nπ ≥ n(1− π) ≥
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