What is unique about a two-tailed hypothesis test?

A two-tailed hypothesis test is distinctive because it does not specify a direction of the effect being tested. This means that the null hypothesis is tested against the possibility of an effect occurring in both directions—either an increase or a decrease. In this type of test, you look for significant deviations in either direction from the null hypothesis, allowing researchers to detect an effect regardless of its nature.

In contrast, a one-tailed test would focus on only one direction (either positive or negative). The two-tailed test is particularly useful when we want to determine if there is any significant effect at all without preconceived notions of whether that effect is positive or negative.

While the other options have their own considerations, they do not accurately describe the nature of a two-tailed test. Certain statistical analyses may require normality or specific directional hypotheses, but those are not defining features of two-tailed hypothesis testing itself.