Question

Will you use a z table or a t table for computing the confidence interval for the slope?

Answer 1

Z table you NEED to:
1. have a normal population AND know it's variance or sample variance
OR
2.assume that is a normal population AND know it's variance
OR
3.have a sample of more than 30 elements, declare the Central Limit Theorem, and use it's variance

T table:
When you have a sample of less than 30 elements, and will use sample variance. Need to assume that the sample came from a normal population.