We want to perform an hypothesis test for a population mean.
In the case that the <u>standard deviation of the population is known</u> and the population distribution is normal, even if the sample is small, <u>we will use a z-test</u>.
The usual case is to not know the standard deviation of the population, in which case a t-test is adequate instead of a z-test, taking into account the degrees of freedom of the sample.
