What is defined as an attribute or characteristic that differs in quality or quantity among different subjects?

The correct choice is the term that refers to an attribute or characteristic that can take on different values or categories among various subjects in a study. In research, a variable is any factor that can be measured or categorized, which can change from one subject to another. This could involve numerical differences, such as age or income, or categorical differences, like gender or race.

Variables are essential in research methodologies as they help define the relationships being studied. For example, if a researcher studies the impact of a particular intervention on different populations, the characteristics that vary between these populations—such as health status or socioeconomic background—are considered variables.

In contrast, other options do not encapsulate this broad definition. Constants remain unchanged throughout a study and are not measured across subjects, while dependent and independent variables refer specifically to the variables involved in cause-and-effect relationships within an experiment. Thus, a general term for differing attributes across subjects is best captured through the definition of a variable.