Dear Fred On 26/11/14 12:15, Fred Labrosse wrote:
I'm obviously missing something somewhere. I'm playing with robust linear fit and experimenting with a number of ways of doing it. So far I have tried GSL and I get the attached result (each line is a point, giving x, y, ordinary least square fit and a robust fit (bisquare)). I plotted that in veusz to verify this was making sense, and it does. Then I tried the fit in veusz, specifically a linear fit (function a + bx). What I get does not make sense (sloping down, going from the first to the last (outlier) points). I restricted the range to ignore the outliers and the fit is much better but still wrong. What is going on? What am I missing?
Did you provide an error bar or uncertainty on your y data? Veusz does a chi-squared minimisation and by default assumes 10% of the data value if not given an uncertainty. This probably is not a good choice for non-logarithmic data. You could try adding a symmetric error to the data value and refitting.
I just tried using 'y,0.1' as the y Data setting (this adds an error bar of 0.1 to your y data set) - this seems to give a fit similar to your least square result.
Best wishes jeremy _______________________________________________ Veusz-discuss mailing list Veuszemail@example.com https://mail.gna.org/listinfo/veusz-discuss