In a random sample of 25 calls made from the U.S.to Japan by callers using Skype during the past week, the average call lasted 21.4 minutes, with a sample standard deviation of 5.1 minutes.Assume the population distribution of call times is normal.Build a 99% confidence interval estimate of the average call time for the population of calls represented by the sample (that is, the population of all calls made from the US to Japan last week by callers using Skype).Report the upper bound for your interval.
A)25.91
B)25.16
C)23.16
D)24.25
E)22.87

Question

Quizplus · Accepted Answer

The upper bound of the 99% confidence interval is calculated using the formula: mean + (t-value * (standard deviation/sqrt(sample size))). With a sample size of 25, the degrees of freedom is 24. The t-value for 99% confidence and 24 degrees of freedom is approximately 2.797. Thus, the upper bound is 21.4 + (2.797 * (5.1/sqrt(25))) = 24.25.

In a Random Sample of 25 Calls Made from the U.S.to