comment from post 3

Nice post.  I don't know about you, but it seems that all the definitions and information regarding when to use the z-test and t-test are the same. There seems to be no discrepancy regarding this.  The z-test is for sample size greater then 30 and when the standard deviation is known.  Otherwise the t-test is used.  I cannot find if there are any exceptions to this rule.  Have you come across any exceptions to the t-test and z-test rules of when to use each hypothesis ? 

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