Question

Construct the requested confidence interval. A meteorologist was interested in the average speed of a thunderstorm in his area. He sampled 13 thunderstorms and found that the average speed at which they traveled across the area was 15 miles per hour with a sample standard deviation of 1.7 miles per hour. Assuming the speed of a thunderstorm is approximately normal, construct a 99% confidence interval for the true average speed of a thunderstorm in his area.

Answer 1

The 99% confidence interval for the true average speed of a thunderstorm in his area is between 13.56 miles per hour and 16.44 miles per hour.

Explanation:

We have the standard deviation for the sample, which means that the t-distribution is used to solve this question.

The first step to solve this problem is finding how many degrees of freedom, we have. This is the sample size subtracted by 1. So

df = 13 - 1 = 12

99% confidence interval

Now, we have to find a value of T, which is found looking at the t table, with 12 degrees of freedom(y-axis) and a confidence level of
`1 - (1 - 0.99)/(2) = 0.995`. So we have T = 3.0545

The margin of error is:

`M = T(s)/(√(n)) = 3.0545(1.7)/(√(13)) = 1.44`

In which s is the standard deviation of the sample and n is the size of the sample.

The lower end of the interval is the sample mean subtracted by M. So it is 15 - 1.44 = 13.56 miles per hour

The upper end of the interval is the sample mean added to M. So it is 15 + 1.44 = 16.44 miles per hour.

The 99% confidence interval for the true average speed of a thunderstorm in his area is between 13.56 miles per hour and 16.44 miles per hour.