##### Statistics: 1001 Practice Problems For Dummies (+ Free Online Practice) Use the t-table as needed and the following information to solve the following problems: The mean length for the population of all screws being produced by a certain factory is targeted to be Assume that you don't know what the population standard deviation is. You draw a sample of 30 screws and calculate their mean length. The mean for your sample is 4.8, and the standard deviation of your sample (s) is 0.4 centimeters. ## Sample questions

1. What is the 95% confidence interval for the population mean? Round your answer to two decimal places.

The formula for the confidence interval for one population mean, using the t-distribution, is In this case, the sample mean, is 4.8; the sample standard deviation, s, is 0.4; the sample size, n, is 30; and the degrees of freedom, n – 1, is 29. That means tn 1 = 2.05.

Now, plug in the numbers: Rounded to two decimal places, the answer is 4.65 to 4.95.

2. What is the 90% confidence interval for the population mean? Round your answer to two decimal places.

The formula for the confidence interval for one population mean, using the t-distribution, is In this case, the sample mean, is 4.8; the sample standard deviation, s, is 0.4; the sample size, n, is 30; and the degrees of freedom, n – 1, is 29. That means that tn – 1 = 1.70.

Now plug in the numbers: Rounded to two decimal places, the answer is 4.68 to 4.92.

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