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T-test or Z-test

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My question is how is it that variance is unknown when they state standard deviation as 10 million?

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50 min in, he gives the explanation that because it’s testing one mean and is a small sample we use the T-test. Gives no mention about standard deviation.

The standard deviation for the sample is known, but not for the population.

Simplify the complicated side; don't complify the simplicated side.

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