Good Morning Class/Professor,

It’s interesting that the topic of this forum is Confidence interval because that about how I feel this week. I think I have the right formula for the right problem and then I try to use excel to double check my work and I use a formula to make sure I’m getting the same thing in excel and then I don’t. It can be a bit frustrating this statistic stuff. Please see the attached spreadsheet. On the left side, you will see 36 prices and this represents the prices of beer per six-pack at your local package store. As you can see on the excel document I get the following data.

Average 8.23

Sigma (sdtdev) = 2.9

E=.95

8.23-.95=\$7.28

8.23+.95=\$9.18

Here is my dilemma and maybe I’m over thinking it. All the numbers seem to be correct in excel, but when I use the formula to double check myself I get a different number for E.

E=Zc Sigma/square root of n E= 1.645 times 2.9/square root of 36 = .79

Please let me know your thoughts and I would love to know why I’m getting two different numbers.

Jason Monaghan