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Jun 26th, 2002, 11:26 AM
#1
Thread Starter
Junior Member
Curve Fitting for 4 parameter logistic equation
I am new to this math forum, and I hope that this is an appropriate place to post this question...
I need to fit data to the "4 parameter logisitic equation" for biological relationships. Essentially the equation is the following:
y = ((A-D) / (1+ ((x/C)^B))) + D
Basically, I'll have an equally sized array of x and y values that are used to solve for variables A,B,C, and D. Right now I need to use one function from a statistical package to solve for this. I'd like to reproduce this function in the language that I am confortable with (VBA), but don't have a clue about how to approach the math! Does anyone know how to do this?!
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Jun 26th, 2002, 05:12 PM
#2
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Jun 27th, 2002, 12:55 PM
#3
Hyperactive Member
I think you could use the Solver feature in Excel to find A, B, C & D for given x & y, although your post suggests you want/need to use statistical functions. Post back if we can help you with the Solver feature.
There are 10 types of people in the world - those that understand binary, and those that don't.
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Jun 27th, 2002, 02:00 PM
#4
Thread Starter
Junior Member
Thanks NotLKH and David for your insight so far. I really appreciate the input. Ideally, I'd like to use pure VB or VBA code. The vendor for the package that I have been using so far said their non-linear regression algorithm is based on the Levenberg-Marquardt method in the publication Numerical Recipes in C (http://chemistry.about.com/gi/dynami...Fbookcpdf.html). Being a dummy in C, C++, and matrix algebra, it all looks like jibberish to me! Does anyone know how to apply the stated method for the model that I originally posted?! From the looks of it, this is starting to be a bear of a project!
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Jun 27th, 2002, 09:18 PM
#5
How many {X, Y} Pairs would you have?
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Jun 28th, 2002, 10:03 AM
#6
Thread Starter
Junior Member
Typically, I would have 18 {X,Y} pairs. The data is made up of triplicate observations at 6 input values. I hope this helps!
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