1. ## Finding t intervals

I understand how to work this problem while finding Z sub alpha/2 but I can't figure out how with t sub alpha/2. The mean is unknown, as needed to be a T interval, i think, but Im not sure how to apply these numbers.
Example:
Find the value:
t sub alpha/2 and n= 18 for the 99% confidence interval for the mean.

thanks

2. Originally Posted by hemoglowen
I understand how to work this problem while finding Z sub alpha/2 but I can't figure out how with t sub alpha/2. The mean is unknown, as needed to be a T interval, i think, but Im not sure how to apply these numbers.
Example:
Find the value:
t sub alpha/2 and n= 18 for the 99% confidence interval for the mean.

thanks
df = n - 1 = 17. $t_{\alpha/2} = t_{0.005} = 2.898$ (using http://www.anu.edu.au/nceph/surfstat...e/tables/t.php, for example).

$\bar{x} - t_{\alpha/2} \frac{s}{\sqrt{n}} < \mu < \bar{x} + t_{\alpha/2} \frac{s}{\sqrt{n}}$

where $\bar{x}$ is the sample mean and s is the sample sd.