All user-defined function equations in STATISTICA graphs are of the same form, e.g., dependent variable = expression. On the left side of the equation, we can specify the dependent variable; for example: y = x^2 + x. However, the left side of the equation is optional (e.g., we can enter the equation as: = x^2 + x or simply as: x^2 + x. On the right side of the equation, the expression can contain Constants, Operators, Functions (including Distributions and their Integrals), and Arguments. For more information on user-defined functions in STATISTICA, see 2D Custom Function Plots - Custom Function tab and 3D Custom Function Plots - Quick tab. In order to fit (and plot) user-defined functions to the data, use the Nonlinear Estimation module. See also, Eps.