This really isn't an XL question.

One key is the sample size...

You may want to look at a beginning statistics textbook.

In article <9135401846fe4@uwe,
"roystonteo via OfficeKB.com" <u48590@uwe wrote:

Dear Experts,

I will need your kind assistance on this. I am very new towards Excel
Functions, hope you have the patience for me. Thank you in advance.

The question is:

A Researcher wants to carry out a test involving the mean for a sample of
size n = 18. She does not know the true value of population standard
deviation, but is reasonably sure that the underlying population is
approximately normally distributed. Should she use a z-test or at-test in
carrying out the analysis?