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swairjo
USA
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 Posted - 02/25/2008 : 5:23:23 PM
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Origin Version (Select Help-->About Origin): 6.0
Operating System:Windows XP Pro
Dear Origin users, I am having difficulty fitting the following data to the following equation. I defined a function and tried to fit with P1 fixed but kept getting a poor fit. Does such a fit exist or should I use a different function in Origin?
IIo No
0 1
0.05 0.9381
0.1 0.70392
0.2 0.68498
0.3 0.60661
0.4 0.55505
0.5 0.51026
0.6 0.48741
0.7 0.37548
0.9 0.32055
1.1 0.26802
2 0.24395
3 0.16626
4 0.09113
5 0.08793
6 0.07
7 0.03532
equation:
IIo=1+(P3-1)*(((P2+P1+No)-sqrt((P2+P1+No)^2-(4*P1*No)))/2*P1)
where IIo is the first column above, No is the second column, P1=2, P2 and P3 are unknown.
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greg
USA
1380 Posts |
Posted - 03/04/2008 : 11:19:51 AM
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I had no difficulty fitting your data with your function in 6.0.
Your function should have three parameters named P1,P2,P3
Your Independent variable should be No
Your Dependent variable should be IIo
Your Definition should be
1+(P3-1)*(((P2+P1+No)-sqrt((P2+P1+No)^2-(4*P1*No)))/2*P1)
and the Form should be expression.
Be sure to save the function before leaving the Edit Function page.
If you are not familiar with fitting, review the topic "Creating a New Function" in the Origin Help file. As with all user defined functions, you must enter good parameter initializations to get a good fit.
Not knowing what to enter, I just assigned 1 to all parameters and clicked the 10 Simplex button once, followed by several clicks of the 10 Iter button to get good convergence.
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fitting nonlinear or where to start?