Question

A geologist examines 18 water samples for iron concentration. The mean iron concentration for the sample data is 0.272 cc/cubic meter with a standard deviation of 0.0851. Determine the 99% confidence interval for the population mean iron concentration. Assume the population is approximately normal. Step 1 of 2: Find the critical value that should be used in constructing the confidence interval. Round your answer to three decimal places.

Answer 1

The critical value that should be used in constructing the confidence interval is T = 2.898.

The 99% confidence interval for the population mean iron concentration is between 0.214 cc/cubic meter and 0.33 cc/cubic meter.

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 = 18 - 1 = 17

99% confidence interval

Now, we have to find a value of T, which is found looking at the t table, with 17 degrees of freedom(y-axis) and a confidence level of
`1 - (1 - 0.99)/(2) = 0.995`. So we have T = 2.898.

The margin of error is:

`M = T(s)/(√(n)) = 2.898(0.0851)/(√(18)) = 0.058`

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 0.272 - 0.058 = 0.214 cc/cubic meter.

The upper end of the interval is the sample mean added to M. So it is 0.272 + 0.058 = 0.33 cc/cubic meter.

The 99% confidence interval for the population mean iron concentration is between 0.214 cc/cubic meter and 0.33 cc/cubic meter.