The random variable whose value serves to determine the action is called the test statistic. A testof the null hypothesis is a course of action specifying the set of values of a test statistic , for which H0 is to be rejected. This set is called the rejection region of the test. A test is completely specified by a test statistic and the rejection region.
Two types of error
Type I error: Rejection of H0 when H0 is true.
Type II error: Nonrejection of H0 when H1 is true.