What defines a two-tailed hypothesis?

A two-tailed hypothesis is characterized by its ability to accommodate potential relationships in any direction, meaning it does not presuppose whether an effect will be an increase or a decrease. This type of hypothesis is used when researchers want to determine if there is a statistically significant difference between groups or a change in a measurement, regardless of the direction of that change. For example, if a researcher is testing a new treatment, a two-tailed hypothesis would evaluate whether the treatment leads to both positive and negative effects compared to a control group.

In contrast, a one-tailed hypothesis only assesses the possibility of an effect occurring in one specific direction, limiting the scope of its investigation. The other options presented do not capture the essence of a two-tailed hypothesis; they either misrepresent its directional intent or imply a focus on variances or qualitative research, which are not applicable in the context of hypothesis directionality.