How do you know when to use Z test or T test?
For most schools & textbooks
- Z test is used when n > 30 or when population standard deviation is known
- t Test is used when underlying population is normal and when n < 30 or when population standard deviation is unknown
You can also ask yourself this question to identify the test used:
Is population standard deviation known?
a) YES –> use z test
b) NO –> Use t test if underlying population is normal