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

[smms <sm...@ho...>]

Thanks Hans for your reply. I had to scale Y for data quantification. 

Date: Fri, 6 Dec 2013 13:48:00 -0800
From: ml-...@n7...
To: sm...@ho...
Subject: Re: Probelm in guessing logistic growth parameters for fitting



	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.


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