Hypothesis Testing -t Test

  How does the teeth statistic differ from the z statistic? What degree of freedoms? Why were there not necessary when d​‌‍‍‍‌‍‍‌‍‌‌‍‍‍‌‍‌‌‌‍​iscussed z scores? And why does one use in-1 rather than just in when calculating the standard deviation as estimated from a sample​‌‍‍‍‌‍‍‌‍‌‌‍‍‍‌‍‌‌‌‍​?  

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