High Quality Content by WIKIPEDIA articles! In statistics, nonlinear regression is a form of regression analysis in which observational data are modeled by a function which is a nonlinear combination of the model parameters and depends on one or more independent variables. The data are fitted by a method of successive approximations. The data consist of error-free independent variables (explanatory variable), x, and their associated observed dependent variables (response variable), y. Each y is modeled as a random variable with a mean a given by a nonlinear function f(x, ). Systematic error may be present but its treatment is outside the scope of regression analysis. If the independent variables are not error-free, this is an errors-in-variables model, also outside this scope.