Question 3: What is a one-tailed or one-sided test?

A one-tailed or one-sided test can be done when we are looking at an estimate of a quantity such as a difference and comparing it to a constant such as zero. Rather than ask whether the quantity could be different from the constant, as in a two-sided test, we ask whether it could be greater than the constant, or less than the constant, specifying which in advance.

For example, instead of testing the null hypothesis that a difference is equal to zero in the population, we might test the null hypothesis that the difference is less than or equal to zero in the population.

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Last updated: 31 July, 2006.