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# Confidence Interval & Sample Size Exercises

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Confidence Intervals and Sample Size

Part I
Suppose the coach of the football team wants to estimate the proportion of the population of fans who support his current starter lineup. The coach wants the estimate to be .04 of the true proportion. Assume a 95 percent level of confidence. The coach estimated the proportion supporting the current starter lineup to be .60.

1. Construct a 95% confidence interval using a sample size of 50, then of 100, then of 1,000.
2. How did changing the sample size affect the size of the interval?
3. What is the error of the estimate for each of these sample sizes?
4. How large of a sample is required for the error of the estimate to be within +.04 of the population proportion?
5. How large would the sample have to be if the coach of the team did not give an estimated proportion?

Other Exercices

Part II
1) You manage a mail order company that advertises that they ship 85% of the orders within two working days. You take a random sample of 20 orders and check to see if indeed the order was shipped within the two day period.
a) What is the probability that 15 or more shipped within the period?
b) What is the probability that all 20 shipped within the period?
c) What is the mean of this distribution?
d) What is the standard deviation of this distribution?
e) What is the probability that less than 20 but more than 14 would be shipped?
f) What is the probability that less than 14 shipped within the period?
g) If less than 14 were shipped within the period what would your conclusion be?
h) What is the probability that between 14 and 18 of the orders were shipped within the two day period?
i) What type of probability distribution is this?
j) Explain how this fits the requirements for this type of distribution.

2) A distribution is a continuous uniform distribution between the values of 2 and 5. Draw the distribution then determine the following:
a) Mean of the distribution
b) P(2< x < 3) = ?
c) P( x > 2.5) = ?
d) P(x > 5) = ?
e) P(2< x < 5) = ?

3) In the standard normal distribution determine the following:
a)  = ?
b)  = ?
c) What is the probability that a value is more than 2.35 standard deviations above the mean?
d) What is the probability that a value is between -1.75 and 1.75 standard deviations from the mean?
e) What is the probability that a value is at least 2.8 standard deviations above the mean?
f) What percent of the data is below z = - 1.43?
g) What z-score represents the 38th percentile?
h) What z-score represents the 99th percentile?
i) Between what two z values will the middle 50% of the data be?

4) According to the College Board, there were 3538 students in Ohio who took at least one of the subject area SAT II tests. These students scored a mean of 651 with a standard deviation of 87 on the SAT I test. This is a normally distributed test.
a) What percent of the students scored below 500 on the test?
b) What percent of the students scored above 700 on the test?
c) Between what two values did the middle 50% of the students score?
d) What value represents the 90th percentile on this test?
e) What is the z value for a student who scored 750 on the test?

5) What are the elements that make up the sampling distribution of the sample mean?
a) How do you determine the mean of the sampling distribution of the sample mean?
b) How do you determine the standard deviation of the sampling distribution of the sample mean?
c) What is another name for this standard deviation of the sampling distribution?
d) Under what two conditions is the sampling distribution of the sample mean considered normal?

6) A population of cereal boxes has a normal weight with a mean of 16 ounces and a standard deviation of 0.2 ounces. If a box is chosen at random what is the probability that it will weigh less than 15.3 ounces? If a sample of 9 boxes is randomly chosen, what is the probability that the mean will be less than 15.3 ounces? If a sample of 25 boxes is randomly chosen, what is the probability that the mean will be less than 15.3 ounces? If you were in charge of filling the boxes and the sample of 25 boxes that you chose had a mean of 15.3 ounces what action would you take?