Quant question -hypothesis test

CFA Level I Quant
#1

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?

#2

The Level I CFA curriculum suggests using a t-test when the sample size is less than 30, the population is (at least assumed to be approximately) normally distributed, and the population’s standard deviation is not known.

1 Like
#3

ok, now i understand, thank you for your nice explain!

#4

My pleasure.