Author 
Topic 

keerthanacivil
6 Posts 
Posted  09/03/2021 : 02:37:20 AM

Hi. I have some data points which are plotted as x axis linear scale and y axis with log scale. On this plot, I need to do curve fitting whose equation is log(y/C)=exp[A*(xB)], A,B,C are parameters. I created a new function. How to write this in the equation?
Kindly help me out 

AmandaLu
356 Posts 
Posted  09/03/2021 : 03:05:54 AM

Hi,
Please refer to this page for how to define a fitting function in Origin: https://www.originlab.com/doc/tutorials/userdeffitfunc
To define your fitting function as explicit type, you will need to transform it to y = C*10^(exp(A*(xB)))
Thanks, Amanda OriginLab Technical Service



AmandaLu
356 Posts 
Posted  09/03/2021 : 04:47:07 AM

Hi,
Parameter’s initial value is very important for fitting with userdefined function. Please make sure you enter proper initial values.
Thanks, Amanda OriginLab Technical Service



keerthanacivil
6 Posts 
Posted  09/03/2021 : 06:01:11 AM

I tried using the parameter values obtained from matlab for fitting in origin. Still the fit is poor in origin. y = C*10^(exp(A*(xB))) In this C is intercept and is same as the unit of Yvalue, presented in log scale. 


AmandaLu
356 Posts 

keerthanacivil
6 Posts 
Posted  09/03/2021 : 07:16:36 AM

Can you share the mail id 


keerthanacivil
6 Posts 
Posted  09/03/2021 : 07:32:02 AM

To which mail I should send 


snowli
USA
1008 Posts 
Posted  09/03/2021 : 08:57:11 AM

U can upload file when you reply the post. See the buttons above the big edit box when you reply. There is upload file button.
Thanks, Snow 


keerthanacivil
6 Posts 
Posted  09/03/2021 : 09:22:38 AM

I am attaching the data in an excel file. log(y/C)=exp[A*(xB)] is the equation to be fitted in origin. I need confidence and prediction bands at 95% with color shading. When I fitted using matlab, I got the parameters as, A=0.242; B=11.06; C=1E11, R2 as 0.91, Adj R2 as 0.91. https://my.originlab.com/ftp/forum_and_kbase/Images/Eboorin.xlsx 


Sam Fang
278 Posts 
Posted  09/06/2021 : 04:49:36 AM

Hi keerthanacivil,
I used your fitted parameters to calculate R2 and Adj R2, and found it is far less than 0.9. This R2 value comes from MATLAB or is calculated by you manually? Did you use a different method to calculate R2?
There are two methods to fit your data in Origin. 1. Transform your y data to log(y). And fit your data with the function (LabTalk script): y=log(C)+exp( A*(xB) )
2. Fit y data directly, but choose Instrumental weight method, and select y as weight data. Here we use weight to force the curve to pass lower y points, otherwise the fit will ignore these points because lower y points contribute less to the residual sum of squares. And the function can be defined as: y=C*10^( exp(A*(xB)) )
Sam OriginLab Technical Services 



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