Once we have identified the independent and dependent variables, our next step in choosing a statistical test is to identify the scale of measurement of the variables. All of the parametric tests that we have learned to date require an interval or ratio scale of measurement for the dependent variable. Many psychologists also apply parametric tests to variables with an approximately interval scale of measurement. It is your decision whether to consider approximately interval or "scale" scores as suitable for parametric tests of means. If you are working with a dependent variable that has a nominal or ordinal scale of measurement, then you must choose a nonparametric statistic to test your hypothesis.

The scale of measurement of the dependent variable helps us to choose the broad category of statistical procedures appropriate for our hypothesis ( nonparametric vs. parametric ). The scale of measurement of the independent variable helps us to determine which statistical procedure within the broad category is appropriate.