What is a two-tailed test?

A two-tailed test is designed to evaluate the possibility of an effect or difference in two directions, meaning it assesses whether the observed data deviates significantly from the null hypothesis in either direction—both higher and lower values. This approach is essential when the research hypothesis does not predict the specific direction of the effect, allowing for the detection of potential changes that could occur in any direction relative to the null hypothesis.

When choosing a two-tailed test, researchers maintain a broader perspective, as they are interested in knowing if the data is significantly different from the null hypothesis without limiting themselves to just one side (either an increase or a decrease). This flexibility is critical in many psychological studies where the nature of the effect is not strictly known beforehand.