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Distribution of Sample Mean in Sampling Distributions

Understand the distribution of the sample mean in normal and non-normal populations. Learn how sample size affects the standard deviation of the sample mean.

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Distribution of Sample Mean in Sampling Distributions

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  1. Chapter 8 Sampling Distributions

  2. Section 8.1 Distribution of the Sample Mean

  3. Objectives • Describe the distribution of the sample mean: normal population • Describe the distribution of the sample mean: nonnormal population

  4. Statistics such as are random variables since their value varies from sample to sample. As such, they have probability distributions associated with them. In this chapter we focus on the shape, center and spread of statistics such as .

  5. The sampling distribution of a statistic is a probability distribution for all possible values of the statistic computed from a sample of size n. The sampling distribution of the sample mean is the probability distribution of all possible values of the random variable computed from a sample of size n from a population with mean μ and standard deviation σ.

  6. Let’s consider a population consisting of {4, 5, 9} - ages of someone’s children If two ages are randomly selected with replacement from the population, identify the sampling distribution of the sample mean.

  7. Sample Distribution of Mean

  8. Sampling Distribution of Mean(condensed) Using this table, let’s calculate the mean of the sample mean: 4(1/9)+4.5(2/9)+5(1/9)+…+9(1/9)=6.0 The mean of the population {4,5,9} is also 6.0!

  9. Because the mean of the sample means is equal to the mean of the population, we conclude that the values of the sample mean do target the value of the population mean. The mean of the sample mean is equal to the population mean µ

  10. Illustrating Sampling Distributions • Step 1: Obtain a simple random sample of size n.

  11. Illustrating Sampling Distributions • Step 1: Obtain a simple random sample of size n. • Step 2: Compute the sample mean.

  12. Illustrating Sampling Distributions • Step 1: Obtain a simple random sample of size n. • Step 2: Compute the sample mean. • Step 3: Assuming we are sampling from a finite population, repeat Steps 1 and 2 until all simple random samples of size n have been obtained.

  13. Objective 1 • Describe the Distribution of the Sample Mean: Normal Population

  14. Parallel Example 1: Sampling Distribution of the Sample Mean-Normal Population The weights of pennies minted after 1982 are approximately normally distributed with mean 2.46 grams and standard deviation 0.02 grams. Approximate the sampling distribution of the sample mean by obtaining 200 simple random samples of size n = 5 from this population.

  15. The data on the following slide represent the sample means for the 200 simple random samples of size n = 5. For example, the first sample of n = 5 had the following data: 2.493 2.466 2.473 2.492 2.471 Note: =2.479 for this sample

  16. Sample Means for Samples of Size n =5

  17. The mean of the 200 sample means is 2.46, the same as the mean of the population. The standard deviation of the sample means is 0.0086, which is smaller than the standard deviation of the population. The next slide shows the histogram of the sample means.

  18. What role does n, the sample size, play in the standard deviation of the distribution of the sample mean?

  19. What role does n, the sample size, play in the standard deviation of the distribution of the sample mean? As the size of the sample increases, the standard deviation of the distribution of the sample mean decreases.

  20. Parallel Example 2: The Impact of Sample Size on Sampling Variability • Approximate the sampling distribution of the sample mean by obtaining 200 simple random samples of size n = 20 from the population of weights of pennies minted after 1982 (μ =2.46 grams and σ=0.02 grams)

  21. The mean of the 200 sample means for n =20 is still 2.46, but the standard deviation is now 0.0045 (0.0086 forn = 5).As expected, there is less variability in the distribution of the sample mean with n =20 than with n =5.

  22. The Mean and Standard Deviation of the Sampling Distribution of Suppose that a simple random sample of size n is drawn from a large population with mean μ and standard deviation σ. The sampling distribution of will have mean and standard deviation The standard deviation of the sampling distribution of is called the standard error of the mean and is denoted .

  23. The Shape of the Sampling Distribution of If X is Normal If a random variable X is normally distributed, the distribution of the sample mean is normally distributed.

  24. Parallel Example 3: Describing the Distribution of the Sample Mean The weights of pennies minted after 1982 are approximately normally distributed with mean 2.46 grams and standard deviation 0.02 grams. What is the probability that in a simple random sample of 10 pennies minted after 1982, we obtain a sample mean of at least 2.465 grams? This is not an individual value!

  25. Solution • is normally distributed with =2.46 and . • . • P(Z > 0.79) = 1 – 0.7852 = 0.2148.

  26. Objective 2 • Describe the Distribution of the Sample Mean: Nonnormal Population

  27. Parallel Example 4: Sampling from a Population that is Not Normal The following table and histogram give the probability distribution for rolling a fair die: μ = 3.5, σ = 1.708 Note that the population distribution is NOT normal

  28. Estimate the sampling distribution of by obtaining 200 simple random samples of size n=4 and calculating the sample mean for each of the 200 samples. Repeat for n = 10 and 30. Histograms of the sampling distribution of the sample mean for each sample size are given on the next slide.

  29. Key Points from Example 4 • The mean of the sampling distribution is equal to the mean of the parent population and the standard deviation of the sampling distribution of the sample mean is regardless of the sample size. • The Central Limit Theorem: the shape of the distribution of the sample mean becomes approximately normal as the sample size n increases, regardless of the shape of the population.

  30. Parallel Example 5: Using the Central Limit Theorem Suppose that the mean time for an oil change at a “10-minute oil change joint” is 11.4 minutes with a standard deviation of 3.2 minutes. • If a random sample of n = 35 oil changes is selected, describe the sampling distribution of the sample mean. (b) If a random sample of n = 35 oil changes is selected, what is the probability the mean oil change time is less than 11 minutes?

  31. Parallel Example 5: Using the Central Limit Theorem Suppose that the mean time for an oil change at a “10-minute oil change joint” is 11.4 minutes with a standard deviation of 3.2 minutes. • If a random sample of n = 35 oil changes is selected, describe the sampling distribution of the sample mean. Solution: is approximately normally distributed with mean = 11.4 and std. dev. = .

  32. Parallel Example 5: Using the Central Limit Theorem Suppose that the mean time for an oil change at a “10-minute oil change joint” is 11.4 minutes with a standard deviation of 3.2 minutes. • If a random sample of n = 35 oil changes is selected, describe the sampling distribution of the sample mean. (b) If a random sample of n = 35 oil changes is selected, what is the probability the mean oil change time is less than 11 minutes? Solution: is approximately normally distributed with mean = 11.4 and std. dev. = . Solution: , P(Z<–0.74) = 0.23.

  33. Designing Elevators When designing elevators, an obviously important consideration is the weight capacity. An Ohio college studetn died when he tried to escape from a dormitory elevator that was overloaded with 24 passengers. The elevator was rated for a capacity of 16 passengers with a total weight of 2500 lb. Weight of adults are changing over time(table below). WE assume a worst-case scenario in which all of the passengers are males. If an elevator is loaded to a capacity of 2500 lb with 16 males, the mean weight of a passenger is 156.25 lb. The distribution is Normal for both males and females.

  34. Find the probablity that 1 randomly selected adult male has a weight greater than 156.25 • Find the probability that a sample of 16 randomly selected adult males has a maximum capacity of 156.25 lb

  35. Approach used for an individual Value • Sample has a mean: • Now use Table V, to find the probabilities…

  36. Section 8.2 Distribution of the Sample Proportion

  37. Objectives • Describe the sampling distribution of a sample proportion • Compute probabilities of a sample proportion

  38. Objective 1 • Describe the Sampling Distribution of a Sample Proportion

  39. Point Estimate of a Population Proportion Suppose that a random sample of size n is obtained from a population in which each individual either does or does not have a certain characteristic. The sample proportion, denoted (read “p-hat”) is given by where x is the number of individuals in the sample with the specified characteristic. The sample proportion is a statistic that estimates the population proportion, p.

  40. Parallel Example 1: Computing a Sample Proportion In a Quinnipiac University Poll conducted in May of 2008, 1745 registered voters nationwide were asked whether they approved of the way George W. Bush is handling the economy. 349 responded “yes”. Obtain a point estimate for the proportion of registered voters who approve of the way George W. Bush is handling the economy.

  41. Parallel Example 1: Computing a Sample Proportion In a Quinnipiac University Poll conducted in May of 2008, 1,745 registered voters nationwide were asked whether they approved of the way George W. Bush is handling the economy. 349 responded “yes”. Obtain a point estimate for the proportion of registered voters who approve of the way George W. Bush is handling the economy. Solution:

  42. Parallel Example 2: Using Simulation to Describe the Distribution of the Sample Proportion According to a Time poll conducted in June of 2008, 42% of registered voters believed that gay and lesbian couples should be allowed to marry. Describe the sampling distribution of the sample proportion for samples of size n = 10, 50, 100. ( ) Note: We are using simulations to create the histograms on the following slides.

  43. Note: The mean of the sample proportions equals the population proportion

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