I swear, every time I think I get it, I end up being completely wrong. Basically, I'm still confused as to how $\displaystyle Z_{0.01}=2.326$, or $\displaystyle Z_{0.025}=1.96$, or this:

http://i29.tinypic.com/wb72vn.jpg

How did $\displaystyle z_{a/2}$ become 2.575? I understand that

$\displaystyle 1-a=0.99$

$\displaystyle a=0.01$

$\displaystyle a/2=0.005$

...but not how that transforms into 2.575. (Worried)