> I need to find a formula for this graph: > > http://qrange.150m.com/misc/plot.htm > > What is good program for nonlinear regression that could help me? > > At the moment, I'm using this formula: > Y*X=22,5 > > or > I*V=22,5 Watt > > but its not very precise.
I think a negative asymptotic function that descends to some lower boundary would fit your data pretty well. I did a test fit of this type of function to some of your data points, and the R^2 figure of 0.9854 shows that it explained 98.54% of the variance.
See the attached plot of your data and the fitted function.
I recommend that you use my NLREG program for the analysis because it can perform general nonlinear regression. You can download a demonstration copy of NLREG from http://www.nlreg.com The demo version should be sufficient for your needs.
Here is the NLREG program to do the fit:
Title "Negative Asymptotic Function: mA = a + b/V"; Variables Vrms, mArms; // Two variables: x and y Parameters a, b; // Two parameters to estimate: a and b Function mArms = a + b/Vrms; // Equation to be fitted to data Plot; // Plot equation and data with grid lines Data; 24 1090 25 960 30 902 35 760 40 690 45 600 50 565 55 520 60 495 64 460