# One-Tailed Test

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## Definition of 'One-Tailed Test'

A one-tailed test is a statistical test in which the hypothesis is directional, meaning that it predicts the direction of the effect. For example, a one-tailed test might be used to test the hypothesis that the mean of a population is greater than a specified value.

One-tailed tests are used when the researcher is only interested in one direction of the effect. For example, a researcher might be interested in testing whether a new drug is effective in reducing pain, but not interested in testing whether the drug is effective in increasing pain. In this case, a one-tailed test would be used to test the hypothesis that the mean of the pain scores for the treatment group is less than the mean of the pain scores for the control group.

One-tailed tests are more powerful than two-tailed tests, meaning that they are more likely to reject the null hypothesis when it is false. However, one-tailed tests are also more likely to make a Type I error, which is the error of rejecting the null hypothesis when it is true.

The decision of whether to use a one-tailed or two-tailed test depends on the research question and the available data. If the researcher is only interested in one direction of the effect, then a one-tailed test should be used. However, if the researcher is interested in both directions of the effect, then a two-tailed test should be used.

One-tailed tests are often used in hypothesis testing. In hypothesis testing, the researcher starts with a null hypothesis, which is the hypothesis that there is no effect. The researcher then collects data and uses a statistical test to test the null hypothesis. If the statistical test rejects the null hypothesis, then the researcher concludes that there is evidence to support the alternative hypothesis.

One-tailed tests are also used in confidence intervals. A confidence interval is an interval of values that is likely to contain the true value of the population parameter. One-tailed confidence intervals are used when the researcher is only interested in one direction of the effect. For example, a one-tailed confidence interval might be used to estimate the mean of a population when the researcher is only interested in estimating the mean if it is greater than a specified value.

One-tailed tests and confidence intervals are important tools for statistical analysis. However, it is important to understand the assumptions and limitations of these tools before using them.

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