The critical region, determined by the significance level $\alpha ,$ is the set of values that will lead to rejecting the null hypothesis $H_0.$ In a standard normal distribution, this region is located in the tails of the distribution. The cutoff value of the region is a critical value given by the $z\text{-}$value of $\alpha .$ The tests of significance — left, right, or two-tail — determine whether there are one or two critical regions.

	Critical Values
Significance Level	Left-Tail Test $H_a:\mu <k$	Two-Tail Test $H_a:\mu \ne k$	Right-Tail Test $H_a:\mu >k$
$\alpha =1\%$	$\text{-}2.326$	$\pm 2.576$	$2.326$
$\alpha =5\%$	$\text{-}1.645$	$\pm 1.960$	$1.645$
$\alpha =10\%$	$\text{-}1.282$	$\pm 1.645$	$1.282$

This table shows the typical significance levels and their corresponding critical values. It is worth noting that the area of the critical region(s) is equal to the significance level $\alpha .$