Module quiz 6.3

An analyst wants to test a hypothesis concerning the population mean of monthly returns for a composite that has existed for 24 months. The analyst may appropriately use:

A. a t-test but not a z-test if returns for the composite are normally distributed.

B. either a t-test or a z-test if returns for the composite are normally distributed.

C. a t-test but not a z-test, regardless of the distribution of returns for the composite.

the correct answer is A

i think we can choose z-test as long as the population is normally distributed, so why not choose B?