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 *H*_{0} 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 *H*_{0} when *H*_{0} is true.

**Type II error**: Nonrejection of *H*_{0} when *H*_{1} is true.