Re: parameters problem in FindFit

• To: mathgroup at smc.vnet.net
• Subject: [mg92873] Re: parameters problem in FindFit
• From: dh <dh at metrohm.ch>
• Date: Thu, 16 Oct 2008 05:05:45 -0400 (EDT)
• References: <gd4dl4\$bg2\$1@smc.vnet.net>

```
Hi,

what do you know about noise in your data. Usually you have some

measurement noise. In this case you need more points to fit 3 parameters.

Further, do you have any reason for the proposed functional form? It

does not seem to fit the data well.

Question is, if the S-shape is noise or real. If it is real, it looks

like a cubic function. If is noise, you need more points.

Mathematica does the best it can. Try plotting the curve with the parameters you

prefer and you will see that you get it completely wrong.

hope this helps, Daniel

gyzhou at 139.com wrote:

> Hi, guys :

>   I get a list like this :

>     lis = {{12.5, 1.146}, {32, 1.145}, {50,

>    1.144}, {69, 1.1424}, {84.6,

>    1.139}}

> My aim is :

>   FindFit[lis,

>    a - (b x^2)/(c + x), {{a, 1.14}, {b, 0.0003}, {c, 300}}, x]

> As we can see, I want this list to be fitted using parameters a, b, c

> around 1.14, 0.0003, 300. However Mathematica gives { b, 10^8} and {c,

>    10^14} far from what I expect.I set "MaxIterations" to less times,

> but it doesn' t make sense.

>

--

Daniel Huber

Metrohm Ltd.

Oberdorfstr. 68

CH-9100 Herisau

Tel. +41 71 353 8585, Fax +41 71 353 8907

E-Mail:<mailto:dh at metrohm.com>

Internet:<http://www.metrohm.com>

```

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