Question

From a random sample of 16 people in a jail, the number of days remaining in each their sentences was noted. The 16 people have a mean remaining sentence of 645 days with a standard deviation of 31 days. Assume the population of remaining sentences is normally distributed. Construct a 95% confidence interval for the mean remaining sentence of all people in the jail. Round final answer to one decimal place.

Answer 1

The 95% confidence interval for the mean remaining sentence of all people in the jail is between 628.5 days and 661.5 days.

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 = 16 - 1 = 15

95% confidence interval

Now, we have to find a value of T, which is found looking at the t table, with 15 degrees of freedom(y-axis) and a confidence level of
`1 - (1 - 0.95)/(2) = 0.975`. So we have T = 2.1315

The margin of error is:

`M = T(s)/(√(n)) = 2.1315(31)/(√(16)) = 16.5`

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 645 - 16.5 = 628.5 days

The upper end of the interval is the sample mean added to M. So it is 645 + 16.5 = 661.5 days

The 95% confidence interval for the mean remaining sentence of all people in the jail is between 628.5 days and 661.5 days.