Re: [Gnuplot-info] Probelm in guessing logistic growth parameters for fitting

[Hans-Bernhard B. <HBB...@t-...>]

On 06.12.2013 21:00, smms wrote:

> So I would appreciate
> if someone could refer me to a good reference of how to make good guessing
> of initial parameters.

That's evidently impossible.  If it there were such a thing as a 
reference manual on how to do it, it would not be called "guessing".

Initial values have to come from your own understanding of the problem. 
  There is no magic bullet.

It says in "help fit" that "Non-linear least-squares fitting is an art!" 
  You're now learning just how true that statement is.

Anyway, your problem is not with the initial values, it's with the 
scaling of the parameters themselves.  You've missed an important lesson 
from "help fit" there.

> a = 1e-7
> b = 10
> k = 0.5
> g(x) = a/(1+b*exp(-k*x))

> fit g(x) 'ss_nici_raw_processed_201.txt' u (1/$1):($2/800**3) via a,b,k
> plot g(x),'ss_nici_raw_processed_201.txt' u (1/$1):($2/800**3):($3/800**3) w
> errorbars not

Hmmm... so why rescale your y axis so heavily, if all that achieves is 
giving your "a" paramter a magnitude that's wildly different from the 
others?

a = 50.0
fit g(x) 'ss_nici_raw_processed_201.txt' u (1/$1):2:3 via a,b,k

would have been equivalent, quicker to type, and with the additional 
benefit of actually using your y errors for the fit.