Confident Interval

Instructions

Use this page as your cover page, and attach your group work behind your work. Your assignment answers should be incomplete and grammatically correct sentences.

1. According to the National Center for Health Statistics (2004), 22. 4% of adults are smokers. A random sample of 300 adults is obtained.

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• (a) Describe the sampling distribution of phat, the sample proportion of adults who smoke.
• (b) In a random sample of 300 adults, what is the probability that at least 50 are smokers?
• c) Would it be unusual if a random sample of 300 adults results in 18% or less being smokers? Explain your answer.

2. A machine at K&A Tube & Manufacturing Company produces a certain copper tubing component in a refrigeration unit. The tubing components produced by the manufacturer have a mean diameter of 0. 75 inches with a standard deviation of 0. 004 inches. The quality-control inspector takes a random sample of 30 components once each week and calculates the mean diameter of these components. If the mean is either less than 0. 748 inches or greater than 0. 752 inches, the inspector concludes that the machine needs an adjustment.

• a) Describe the sampling distribution of the sample means diameter for a random sample of 30 such components.
• (b) What is the probability that, based on a random sample of 30 such components, the inspector will conclude that the machine needs an adjustment when, in fact, the machine is correctly calibrated?

3. In a random sample of 678 adult males 20 to 34 years of age, it was determined that 58 of them have hypertension (high blood pressure). Source: The Centers for Disease Control.

• (a) Obtain a point estimate for the proportion of adult males 20 to 34 years of age who have hypertension.
• (b) Construct a 95% confidence interval for the proportion of adult males 20 to 34 who have hypertension. Interpret the confidence interval. You wish to conduct your own study to determine the proportion of adult males 20 to 34 years old who have hypertension.
• (c) What sample size would be needed for the estimate to be within 3 percentage points (interval length is 0. 06) with 95% confidence if you use the point estimate obtained in part (a)?
• (d) What sample size would be needed for the estimate to be within 3 percentage points with 95% confidence if you don’t have a prior estimate (use phat=. )?

4. A random sample of 60 married couples who have been married 7 years was asked the number of children they have.

• (a) Obtain a point estimate for the mean and standard deviation number of children of all couples who have been married 7 years.
• (b) What is the shape of the distribution of the sample mean? Why?
• (c) Compute a 95% confidence interval for the mean number of children of all couples who have been married 7 years. Interpret this interval.
• (d) Compute a 99% confidence interval for the mean number of children of all couples who have been married 7 years. Interpret this interval.
• (e) You wish to conduct your own study to determine the mean number of children of all couples who have been married for 7 years. What sample size would be needed for the estimate to be within 0. 25 (interval length is 0. 5) with 99% confidence if you use the point estimates obtained in part (a)?

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