We don't know any measurement of a psychological variable that definitely has the characteristics of an interval scale. Many concepts measured by psychologists produce numbers that clearly contain more information than the ordinal scale, but demonstrating that the intervals are equal requires assumptions that are not universally accepted. However, most researchers treat these measurements as if they were interval data, although they cannot demonstrate that equal intervals indicate equal quantities of the variable being measured. Therefore, variables such as intelligence, depression, happiness, sociability and many others are generally treated as an interval scale variable.
The dependent variable is the variable that is being tested and measured in an experiment, and is “dependent” on the independent variable. An example of a dependent variable is symptoms of depression, which depend on the independent variable (type of therapy). For example, a 5-year longitudinal study showed a temporal association between perceived levels of loneliness and the severity of subsequent depression, such that loneliness predicted increases in depressive symptoms independently of other factors such as demographic variables, objective social isolation, stress, disposition, negativity or social support (Cacioppo et al. A central aspect of the question of structure is whether depression represents discrete types or occurs in a continuum.
For example, the statistical relationship between whether or not a depressed person receives psychotherapy and the number of depressive symptoms he has reflects the fact that psychotherapy (the independent variable) causes reduction of symptoms (the dependent variable).