I Curve fitting

@ sounouhid : One cannot answer to your question without a clear description of your problem. What kind of "theoretical model" are you talking about ? What kind of data ? And more details on the context of the problem are essential. The fitting methods are numerous and depends on the context. A whole book should not large enough to answer if the problem is not restricted. For example, some procedures of fitting to several models are available in : https://fr.scribd.com/doc/14674814/Regressions-et-equations-integrales .(paper in French with some parts in English. All procedures are understandable in English)