Originally Posted by
meymathis I have not heard of specification hours. It looks like you are supposed to calculate the confidence interval with 95% confidence level.
The Central Limit Theorem (which the general rule of thumb says is ok to invoke for $\displaystyle N\geq 30$) tells us that $\displaystyle \frac{\bar{X}-\mu}{S \sqrt{N}}$ is approximately normally distributed with mean 0 and std dev 1.
So then the confidence interval is given by $\displaystyle \bar{X}\pm z_{\alpha/2}S/\sqrt{N}$ where $\displaystyle z_{\alpha/2}$ is defined by $\displaystyle \mathrm{P}(-z_{\alpha/2}\leq Z \leq z_{\alpha/2})=1-\alpha$ (Z is from the standard normal distribution). I'm assuming that you have already seen where these formulas came from.
So from a standard normal distribution table $\displaystyle z_{\alpha/2}=1.96$. So just plug away.