When the alternative hypothesis  is one-sided like  or, then the rejection region is taken only on one side of the sampling distribution. It is called one-tailed test or one-sided test. When  is one-sided to the right like, the entire rejection region equal to  is taken in the right end of the sampling distribution.
            The test is called one-sided to the right. The hypothesis  is rejected if the calculated value of a statistic, say  falls in the rejection region. The critical value is which has the area equal to  to its right. The rejection region and acceptance region are shown in Figure. The null hypothesis  is rejected when (calculated).

            If the alternative hypothesis is one-sided to the left like, the entire rejection region equal to  is taken on the left tail of the sampling distribution. The test is called one-sided or one-tailed to the left. The critical value is  which cuts off the area equal to  to its left. The critical region is  and is shown in Figure.
For some important values of, the critical values of  for two-tailed and one tailed tests are given below:

Critical Value of

	Two-sides test	One-sided test	One-sided to the left
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