One-tailed test is a method of hypothesis testing where the alternative hypothesis specifies in which direction the parameter differs from the value stated in the null hypothesis. That is, the alternative hypothesis states if the parameter is above or below the value in the null hypothesis. One-tailed hypothesis testing is widely used in quantitative research when the direction of the population parameter's deviation from the value in the null hypothesis can be predicted in advance or when researchers are interested in results in a specific direction. This entry explains one-tailed tests in connection to other aspects of hypothesis testing and describes contexts in which one-tailed tests are the appropriate type of hypothesis testing.