[R] Biexponential Fit

[Jonas Weickert]

Thank you! Now it's working.

Peter Dalgaard schrieb:
> Jonas Weickert wrote:
>> Hi,
>>
>> I want to do a biexponential Fit, i.e.
>>
>> y ~ A1*exp(k1*x) + A2*exp(k2*x)
>>
>> Is this possible? I tried nls() but it stopped with several 
>> (different) errors. I'm using y and x as simple vectors and the 
>> formula for nls() exactly as mentioned above.
> 
> Yes, it is possible, with some reservations. There is even a 
> self-starting model for it (SSbiexp), at least if k1,k2 are negative.
> 
> The reservations are that you need good data and good starting values 
> for the fit. The model is theoretically unidentifiable because you can 
> switch 1 and 2 and get the same model, which becomes an issue if you 
> start too fra from the optimum. Worse, if k1 and k2 are similar, the 
> whole system becomes ill-conditioned. There's a rule of thumb requiring 
> a factor of 5 or more between k1 and k2, for pharmacokinetic applications.
> 
>> Thanks a lot!
>>
>> Jonas
>>
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> 
>