comment from post 4

Z tests and t tests are formulas used in hypothesis testing.   Hypothesis testing is a way to figure out if results from a test are valid and repeatable.   Z tests are used when your data is normally distributed, the standard deviation is known, and the sample size is greater than 30. T tests compare the two means of tests and tells you if they are different from each other and how significant the differences are, meaning could they have happened by chance.   T tests are more commonly used and do not require the standard deviation be known.  

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