DEFINITION of 'OneTailed Test'
A statistical test in which the critical area of a distribution is onesided so that it is either greater than or less than a certain value, but not both. If the sample that is being tested falls into the onesided critical area, the alternative hypothesis will be accepted instead of the null hypothesis. The onetailed test gets its name from testing the area under one of the tails (sides) of a normal distribution, although the test can be used in other nonnormal distributions as well.
INVESTOPEDIA EXPLAINS 'OneTailed Test'
An example of when one would want to use a onetailed test is in the error rate of a factory. Let's say a label manufacturer wants to make sure that errors on labels are below 1%. It would be too costly to have someone check every label, so the factory selects random samples of the labels and test whether errors exceed 1% with whatever level of significance they choose. This represents the implementation of a onetailed test.

TwoTailed Test
A statistical test in which the critical area of a distribution ... 
Standard Deviation
1. A measure of the dispersion of a set of data from its mean. ... 
Null Hypothesis
A type of hypothesis used in statistics that proposes that no ... 
Type I Error
A type of error that occurs when a null hypothesis is rejected ... 
Type II Error
A statistical term used within the context of hypothesis testing ... 
Hypothesis Testing
A process by which an analyst tests a statistical hypothesis. ...

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