# Thread: formula for confidence interval for a mean

1. ## [RESOLVED]formula for confidence interval for a mean

In the following formula for creating a confidence interval for a mean where do I get $\displaystyle Z_{\alpha/2}$ from? $\displaystyle \bar{X}\pm Z_{\alpha/2}\frac{\sigma}{\sqrt{n}}$

I know it has something to do with using z-scores and I've been trying to work backwards from examples. For example a confidence interval of 90% should have $\displaystyle Z_{\alpha/2}=1.645$ but when I look up 0.9 I get 0.8159.

2. Z table - Normal Distribution

Go to the second graph and type in 0.9 in 'shaded area' and click on 'between' as $\displaystyle \frac{\alpha}{2}$ implies a two tail interval