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## genfit problem about number of parameters

The error message says the number of fitting parameters and guess values must be the same, but they do !!!

Please, take a look to the file attached, this is a short programm.

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## Re: genfit problem about number of parameters

Not exactly sure what you mean by "The combination of these three functions gives a family of non-asymptotic sigmoid functions". Which kind of "combination"?

If you have a simple additive combination in mind, you would have to do it that way, nut the result sure is not satisfactory (and very sensitive wrt the guess values):

But if you want to say that you would like to split your curve into three parts and fit a different function type to each part, then you would have to do the splitting yourself and fit three times.

BTW, Here's a function which seems to fit your data pretty well:

6 REPLIES 6

## Re: genfit problem about number of parameters

Not exactly sure what you mean by "The combination of these three functions gives a family of non-asymptotic sigmoid functions". Which kind of "combination"?

If you have a simple additive combination in mind, you would have to do it that way, nut the result sure is not satisfactory (and very sensitive wrt the guess values):

But if you want to say that you would like to split your curve into three parts and fit a different function type to each part, then you would have to do the splitting yourself and fit three times.

BTW, Here's a function which seems to fit your data pretty well:

## Re: genfit problem about number of parameters

Yeah, I was talking about "linear combinations of functions" , sorry.

Thanks, you helped me to fix this error, now I can fit with genfit.

I think mathcad is not good for fitting points if they are not well close to a mathematical curve, but experimental points are random up and down from such a curve.

I was hopping to fit experimental points to non asymptotic sigmoid curves like this https://stats.stackexchange.com/questions/398551/logistic-function-with-a-slope-but-no-asymptotes

(because some chemical law predicts this) you see there are 5 paramenters, I also added a sixth parameter x0  in this form: instead x I put (x-x0) hopping the fitting process could find the best x0, but the outcome was ugly.

SigmaPlot can fit this without problem (sorry for comparison) but at the moment I haven't this software.

Thank you anyway.

## Re: genfit problem about number of parameters

Mathcad often is very sensible with respect to the guess values which will have to be chosen with great care.

You may want to have a look at the eval version of CurveExpert Pro. Even in demo w/o the various export options mode its quite useful for curve fitting. It has a lot of predefined function types which it tries on the data and you may also define your own.

## Re: genfit problem about number of parameters

+1 for CurveFit Pro.  One thing I have been using it for lately is reverse engineering a few older nomographs that are used in my type of civil engineering, but for which I cannot find the equations that were used to build them.  I extract data points from the curves with a scale, drop them into CurveFit Pro, and find the best fit equation that I can then use in Mathcad, Excel, or one of my programmable HP calculators.

## Re: genfit problem about number of parameters

Could not find "CurveFit Pro" in a first quick search on the net. Did you mean "CurveExpert Pro" which I mentioned above?

## Re: genfit problem about number of parameters

Werner...

You are correct.  I brain-freezed the name.

Fred

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