# Z Table Rookie Question

#### AnnVascol

##### New Member
We've been told to find critical value that corresponds to 80% confidence level.

I dropped 10% from each tail since it's symmetric. Am I right in proceeding by finding where 0.90 is in the table to find out the right limit and then minus that to find the left-end point hoping it leaves 80% in the middle? I am using this z table, can someone tell me if I am proceeding the right way and if not, how to use the Z Table?

#### obh

##### Active Member
Hi Ann,

Yes, 1-0.2/2=0.9 and 1-0.9=0.1

When having a computer, you don't need to use z-table (unless your teacher asks...)
If you estimate the standard deviation from a sample then you need to use the T distribution. T(1-α/2)
If you calculate the CI of the average then the standard deviation of the average is "standard deviation of the population"/sqrt(n)
You may look at the calculated charts on http://www.statskingdom.com/40_confidence_interval.html