Question

One​ year, the mean age of an inmate on death row was 39.2 years. A sociologist wondered whether the mean age of a​ death-row inmate has changed since then. She randomly selects 32 ​death-row inmates and finds that their mean age is 37.6​, with a standard deviation of 9.89.8. Construct a​ 95% confidence interval about the mean age. What does the interval​ imply?

Answer 1

The 95% confidence interval about the mean age is between 17.6 years and 57.6 years.

This means that we are 95% sure that the mean age of an inmate in the death row is in this interval. 39.2 is part of this interval, which implies that the mean age of a​ death-row inmate has not changed since then.

Explanation:

We have the sample standard deviation, so we use the t-distribution 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 = 32 - 1 = 31

95% confidence interval

Now, we have to find a value of T, which is found looking at the t table, with 31 degrees of freedom(y-axis) and a confidence level of
`1 - (1 - 0.95)/(2) = 0.975`. So we have T = 2.0395

The margin of error is:

M = T*s = 2.0395*9.8 = 20

In which s is the standard deviation of the sample.

The lower end of the interval is the sample mean subtracted by M. So it is 37.6 - 20 = 17.6 years

The upper end of the interval is the sample mean added to M. So it is 37.6 + 20 = 57.6 years.

The 95% confidence interval about the mean age is between 17.6 years and 57.6 years.

This means that we are 95% sure that the mean age of an inmate in the death row is in this interval. 39.2 is part of this interval, which implies that the mean age of a​ death-row inmate has not changed since then.