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A doctor wants to estimate the mean HDL cholesterol of all 20- to 29-year-old females. How many subjects are needed to estimate the mean HDL cholesterol within 3 points with 99% confidence assuming s = 18.3 based on earlier studies? Suppose the doctor would be content with 95% confidence. How does the decrease in confidence affect the sample size required? Click the icon to view a partial table of critical values. .. A 99% confidence level requires subjects. (Round up to the nearest subject.) A 95% confidence level requires subjects. (Round up to the nearest subject.) How does the decrease in confidence affect the sample size required? O A. Decreasing the confidence level decreases the sample size needed. O B. Decreasing the confidence level increases the sample size needed. OC. The sample size is the same for all levels of confidence.

Determine the t-value in each of the cases. Click the icon to view the table of areas under the t-distribution. (a) Find the t-value such that the area in the right tail is 0.05 with 28 degrees of freedom. (Round to three decimal places as needed.) (b) Find the t-value such that the area in the right tail is 0.025 with 23 degrees of freedom. (Round to three decimal places as needed.) (c) Find the t-value such that the area left of the t-value is 0.25 with 10 degrees of freedom. (Hint: Use symmetry.] (Round to three decimal places as needed.) (d) Find the critical t-value that corresponds to 98% confidence. Assume 7 degrees of freedom. (Round to three decimal places as needed.)

A simple random sample of size n = 40 is drawn from a population. The sample mean is found to be x = 121.8 and the sample standard deviation is found to be s = 12.2. Construct a 99% confidence interval for the population mean. The lower bound is (Round to two decimal places as needed.) The upper bound is (Round to two decimal places as needed.)