T Test Vs Z Test

The difference between a t-test and a z-test comes down to what you know about your data: the sample size and the population standard deviation.

Use a t-test when:

• The sample size is small (usually n ≤ 30).
• The population standard deviation is unknown.
• You still want to compare means (sample vs. population or between two samples).

Use a z-test when:

• You know the population standard deviation.
• Your sample size is large (typically n > 30).
• The data is roughly normally distributed.