Single equation regression models - The dependent variable, Y, is expressed as a linear function of one or more explanatory variables, the Xs Assumption the cause-and-effect relationship, if any, between Y and the Xs is unidirectional: explanatory variables are the cause; the dependent variable is the effect• Single equation regression models: ——The dependent variable, Y, is expressed as a linear function of one or more explanatory variables, the Xs. Assumption the cause-and-effect relationship, if any, between Y and the Xs is unidirectional: ·explanatory variables are the cause； ·the dependent variable is the effect