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Kathy_Wang
China
159 Posts |
 Posted - 03/13/2014 : 03:11:32 AM
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Hi,
Firstly could you please verify what your fitting function looks like? It seems to be y=1-exp(-(a*x)^b) but you define the constraint in a different equation that instead of a*x you used a^x.
In addition, it is actually not possible for a function in form y = 1-exp(-(a*x)^b) nor y = 1-exp(-(a^x)^b) to go through point (0.5, 0), unless a equals 0 but then the value of independent variable x do not affect the dependent variable y any more.
Also, in Origin the constraint need to be defined in a linear mode, i.e. the parameters in the defined constraint should be in linear relationship such as 0.5*a + b = 0 but the constraint you defined is a nonlinear one. In that case, you may need to use a different method to make sure the fitted curve go though certain point, two available options are:
a. add an offset to the fitting function, for example change it to y = exp(-(0.5*a)^b) - exp(-(a*x)^b) and fit it directly without defining a constraint and it definitely goes through (0.5, 0)
b. use weighting, you may refer to this tutorial for details.
Kathy
Originlab |
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Constraint for fitting function
