 # Samples The means of these samples

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Samples The means of these samples
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Example Random samples of size n =2 are drawn from a finite population that consists of the numbers 2, 4, 6 and 8 without replacement. a-) Calculate the mean and the standard deviation of this population b-) List six possible random samples of size n=2 that can be drawn from this population and calculate their means. c-) Use the results in b-) to construct the sampling distribution of the mean. d-) Calculate the standard deviation of the sampling distribution.

Example for Correction Factor:
What is the value of the finite population correction factor when a-) n= 20 and N=200 ? b-) n= 20 and N= 2000 ?

Example 2: Tuition Cost The mean tuition cost at state universities throughout the USA is 4,260 USD per year (2002 year figures). Use this value as the population mean and assume that the population standard deviation is 900 USD. Suppose that a random sample of 50 state universities will be selected. A-) Show the sampling distribution of x̄ (where x̄ is the sample mean tuition cost for the 50 state universities) B-) What is the probability that the random sample will provide a sample mean within 250 USD of the population mean? C-) What is the probability that the simple random sample will provide a sample mean within 100 USD of the population mean?

Example 4: Suppose we have selected a random sample of n=36 observations from a population with mean equal to 80 and standard deviation equal to 6. Q: Find the probability that x̄ will be larger than 82.

Example 5: Ping-Pong Balls
The diameter of a brand of Ping-Pong balls is approximately normally distributed, with a mean of 1.30 inches and a standard deviation of 0.04 inch. If you select a random sample of 16 Ping-Pong balls, A-) What is the sampling distribution of the sample mean? B-) What is the probability that sample mean is less than 1.28 inches? C-) What is the probability that sample mean is between 1.31 and 1.33 inches? D-) The probability is 60% that sample mean will be between what two values, symmetrically distributed around the population mean?

Example 6: s Time spent using per session is normally distributed, with a mean of 8 minutes and a standard deviation of 2 minutes. If you select a random sample of 25 sessions, A-) What is the probability that sample mean is between 7.8 and 8.2 minutes? B-) What is the probability that sample mean is between 7.5 and 8.0 minutes? C-) If you select a random sample of 100 sessions, what is the probability that sample mean is between 7.8 and 8.2 minutes? D-) Explain the difference in the results of (A) and (C).

Types of Survey Errors Coverage error Non response error
Sampling error Measurement error Excluded from frame Follow up on nonresponses Random differences from sample to sample Bad or leading question

Z X Population Distribution Sampling Distribution
Standard Normal Distribution ? ? ? ? ? ? ? ? ? ? Sample Standardize ? ? X Z

Sampling Distribution Properties
As n increases, decreases Larger sample size Smaller sample size

Sampling Distribution Properties
Normal Population Distribution (i.e is unbiased ) Normal Sampling Distribution (has the same mean) Variation:

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 For normal population distributions, the sampling distribution of the mean is always normally distributed

Exercise - 1 A package-filling process at a Cement company fills bags of cement to an average weight of µ but µ changes from time to time. The standard deviation is σ = 3 pounds. A sample of 25 bags has been taken and their mean was found to be 150 pounds. Assume that the weights of the bags are normally distributed. Find the 90% confidence limits for µ.

Exercise - 3 An economist is interested in studying the incomes of consumers in a particular region. The population standard deviation is known to be \$1,000. A random sample of 50 individuals resulted in an average income of \$15,000. What is the upper end point in a 99% confidence interval for the average income?

Exercise - 4 An economist is interested in studying the incomes of consumers in a particular region. The population standard deviation is known to be \$1,000. A random sample of 50 individuals resulted in an average income of \$15,000. What is the width of the 90% confidence interval?

Exercise - 5 The head librarian at the Library of Congress has asked her assistant for an interval estimate of the mean number of books checked out each day. The assistant provides the following interval estimate: from 740 to 920 books per day. If the head librarian knows that the population standard deviation is 150 books checked out per day, and she asked her assistant for a 95% confidence interval, approximately how large a sample did her assistant use to determine the interval estimate?