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

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

I am having difficulty in guessing initial parameters values for logistic fit
plotting to my data set. The parameters I used actually from somebody else.
Going through gnuplot helpf fit did not help at all. So I would appreciate
if someone could refer me to a good reference of how to make good guessing
of initial parameters. Also if you happen to find a good values for a nice
fit, can you please explain your basis of choosing different parameters?
Included below are graph, data and codes.

This initial values are getting very frustrating and hence many thanks for
your any feedback in advance. 

<http://gnuplot.10905.n7.nabble.com/file/n17874/lgfit.png> 
x       y       sd
2.50	9.04	0.03
2.25	9.06	0.05
1.11	9.12	0.19
0.71	9.97	0.18
0.60	11.36	0.24
0.56	12.44	0.55
0.50	14.23	0.37
0.43	16.93	1.20
0.38	19.18	1.12
0.32	24.83	2.26
0.30	28.87	1.39
0.25	34.23	2.07
0.21	39.52	0.53
0.16	44.10	1.81
0.13	49.73	1.19

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





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