The word independent variable comes from mathematics. In mathematics, y=f(x). In this equation, the independent variable is X and the dependent variable is Y. When this equation is applied to psychological research, the independent variable is the factor or condition that guides the change of the cause variable, so the independent variable is regarded as the cause of the dependent variable. Independent variables can be divided into continuous variables and category variables. If the independent variable manipulated by the experimenter is a continuous variable, the experiment is a functional experiment. If the independent variable manipulated by the experimenter is a category variable, the experiment is a factor type. In psychological experiments, an obvious problem is to have an organism as a subject to respond to stimuli. Obviously, the stimulus variables here are independent variables.