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Screensize: 640 x 480 800 x 600 1024 x 768 1280 x 1024 UserName: Password: Anti-Spam Code: Format Mode: Basic  Help  Prompt  Format: Font Andale Mono Arial Arial Black Book Antiqua Century Gothic Comic Sans MS Courier New Georgia Impact Lucida Console Script MT Bold Stencil Tahoma Times New Roman Trebuchet MS Verdana   Size 1 2 3 4 5 6   Color Black Red Yellow Pink Green Orange Purple Blue Beige Brown Teal Navy Maroon LimeGreen Forum: Origin Forum Subject: Message: * HTML is OFF * Forum Code is ON Smilies Dear All, I am new to ORIGIN and therefore I hope you can help/explain me. I need to perform a simple 3rd degree polynomial fit to my data: [b]X[/b] 15.875 9.525 3.175 -3.175 -10.525 -16.875 -23.225 [b]Y[/b] 0.411 0.555 0.67 0.728 0.727 0.664 0.547 [b]err_Y[/b] 1E-3 0.002 0.002 0.002 0.002 0.002 0.002 The fit has to be done on Y, considering the err_Y. To do that, I follow the recommendations of using "instrumental" and "reduce Ci-sq" (as options). By doing that, I obtained the fit coefficients and their related uncertainties. Now, I increased the error_y by 100 (i.e., 2 order of magnitude). I am expecting that this has an impact on the uncertainties of the coefficients. However, nothing change and the uncertainties stayed the same. What am I doing wrong/not getting? Why the uncertainties on the fit coefficients do not change if the error on the points increased? Thanks for your help! Christian Fedon   Check here to subscribe to this topic.