One thing that I was a bit fuzzy on before our review today was the difference between a hypothesis test for when the variance is known versus when the variance is unknown. For a known variance, the test stat is z=xbar-mu/(sdev/sqrt(n)), where z~N(0,1). For an unknown variance, the test stat is t=xbar-mu/(s/sqrt(n)),where s is the sample standard deviation, and t~T_(n-1).