Question

A geologist examines 14 geological samples for iron concentration. The mean iron concentration for the sample data is 0.181 cc/cubic meter with a standard deviation of 0.0318. Determine the 90% 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 90% confidence interval for the population mean iron concentration is between 0.167 cc/m³ and 0.195 cc/m³.

Explanation:

We have that to find our
`\alpha` level, that is the subtraction of 1 by the confidence interval divided by 2. So:

`\alpha = (1-0.9)/(2) = 0.05`

Now, we have to find z in the Ztable as such z has a pvalue of
`1-\alpha`.

So it is z with a pvalue of
`1-0.05 = 0.95`, so
`z = 1.645`

Now, find M as such

`M = z*(\sigma)/(√(n))`

In which
`\sigma` is the standard deviation of the population and n is the size of the sample.

`M = 1.645*(0.0318)/(√(14)) = 0.0140`

The lower end of the interval is the sample mean subtracted by M. So it is 0.181 - 0.0140 = 0.167 cc/m³.

The upper end of the interval is the sample mean added to M. So it is 0.181 + 0.0140 = 0.195 cc/m³.

The 90% confidence interval for the population mean iron concentration is between 0.167 cc/m³ and 0.195 cc/m³.