Biexponential trend or fit

[benaviner]

Hello experts!

I need your help with calculating the parameters of biexponential (two term exponent) fit of my data. Or in other words, getting the parameters of a biexponential trendline of my data, known to be following this equation:

y= A1*e^(b1*x) + A2*e^(b2*x)

I am more interested in the b1, b2 parameters, if that helps.
The data is in a few hundred rows in two columns (x & y).

Excel seems to only give parameters of a simple exponential fit, e.g.

y = 5e^-0.66x , or y=A*e^(b*x)

I have even failed to find an Excel add-in to do the job.

Will greatly appretiate it if you could dedicate your time to solve this problem!

Thank you,
Ben

 

[Herbert Seidenberg]

Excel 2010 Tables
Curve Fitting
Double Exponential
http://c3017412.r12.cf0.rackcdn.com/04_21_11b.xlsx
If you get *.zip, don't unzip, just rename *.xlsx

 

[benaviner]

Dear Herbert,

Thank you for dedicating your time to my post.

1. Did you use ZunZun.com engine in order to find the coefficients? How does one enter the data to this engine in order for it to calculate? Can the results be automatically imported to Excel?

2. I am seeking for a way to make excel calculate the coefficients of a bi-exponential curve by itself (perhaps with an Add-in or VBA programming?), as it successfully does for a single exponential curve (e.g. using the function LOGEST, but with the number "e" as I described in the original post, or as it does while calculating an exponential trend line: please see image, R square shows how inaccurate this trend line is http://www.screencast.com/t/AdD5D1Mr4UdO).

Respectfully,
Ben

 

[Herbert Seidenberg]

ZunZun.com (free) or Mathematica ($$$, import, export) is the way to go.
Play with it.

 

[wigi]

benaviner

Did you try Excel Solver? Might be possible if you start with reasonable starting values.

Wigi

 

[zunzun]

...in other words, getting the parameters of a biexponential trendline of my data, known to be following this equation:

y= A1*e^(b1*x) + A2*e^(b2*x)

The zunzun.com fitting page for this equation is here:

http://zunzun.com/Equation/2/Exponential/Double Exponential/

James Phillips
http://zunzun.com
zunzun@zunzun.com
2548 Vera Cruz Drive
Birmingham, AL 35235 USA

 

[benaviner]

wigi,

Thank you! I did not know Solver before, and indeed succeeded to find a solution using it!

Herbert,

Zunzun is useful for finding a single solution for a set of data, but I am not aware of a way it can be used "in-house" by excel.

Respectfully,
Ben

 

[benaviner]

Dear All,

Unfortunately Excel Solver is still too limited in its method of solving the issue at hand, and it is not reasonable to run it for 1000s of times...
Could this fit be calculated using a VBA code that can be integrated/used within excel?

Thank you ,
Ben