Question

How do I know whether to use the Z-table or T-table?

Answer 1

It's based on if you know the standard deviation of the population or not.

Explanation:

Whenever you collect data and want to make a confidence interval, you want to check first what you know. Now most of the time you probably don't know what the population standard deviation, though you could always use a standard deviation obtained from a preliminary study as the population standard deviation. When you do know the population standard deviation, you can define the confidence interval as a 1-sample z test, and use z tables to construct the confidence interval.

If you don't know the population standard deviation, then you define the confidence interval as a 1-sample t test, and you would use t tables to construct a confidence interval.