One of two fit-engines can be chosen from the Fitting menu bar. The Simple Fit-engine uses Scipy's leasqr and the Complex Fit-Engine is a custom optimizer that provides a better chance to find the global minimum of the chi2 but that requires longer computation time. In order to set a data to a control panel (FitPage), see the "DataLoader Help". Once a data set to the FiPage, select a model from the combo box. The default parameters of the model will be display. Set initial parameters if need. Check and uncheck parameters to fit/fix. Click the 'Fit' button. When the fitting is finished, the resultant parameter values will be displayed with the errors. If a error is missing, it generally means that the corresponding parameter is not very depending on the model. The chisq/Npt_fit and the plot associated with the fit operation will be also updated.