8 T-test
In previous discussions, we examined hypothesis testing using z-tests, which we apply when we know the population’s standard deviation. However, real-world scenarios often present a different challenge: the population’s standard deviation is frequently unknown. This common occurrence requires us to adjust our approach.
When decision-makers encounter an unknown population standard deviation, they estimate this crucial parameter using the sample’s standard deviation. This estimation process introduces additional uncertainty into our calculations, necessitating the use of a different statistical tool: the t-test.
OpenLearn - Data analysis: hypothesis testing 
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