Chemistry student help

[Camilo Mejia P.]

Hi everyone. I'm a chemistry student.

I have some questions about COPASI. I've used it before. I used it to determine the k-value from my data ([DPPH] vs. Time) using a parametric fit, and I obtained the respective values.

My question is, what does the standard deviation in the results mean? Could you give me the definition?
My other question is, when I write the parametric limits, what does the "Start Value" mean? Is there an extra way to determine it, or is it through trial and error?

And could you please tell me how to use the validation option? Thanks.

 

This is my result, using Evo programming. with: Number of generations 200 y population size of 20

[Sahle, Dr. Sven]

Dear Camilo, 

Am 18.12.2025 um 19:45 schrieb Camilo Mejia P. <yeine...@gmail.com>:

Hi everyone. I'm a chemistry student.

I have some questions about COPASI. I've used it before. I used it to determine the k-value from my data ([DPPH] vs. Time) using a parametric fit, and I obtained the respective values.

My question is, what does the standard deviation in the results mean? Could you give me the definition?

The standard deviation is an approximative measure for how accurate the specific parameter value could be estimated from the data. Conceptually the idea is that you could repeat the measurements and due to measurement errors you would get slightly different data resulting in slightly different parameter values after fitting. The SD tries to estimate how big the changes in the fitted parameter values would be. 

Technically Copasi calculates it as follows: From the objective function Copasi calculates the Fisher information matrix, and from that (by inverting) the correlation matrix. The diagonal elements of the correlation matrix indicate how the measurement error in the data translates into an error in the estimated parameters. 

My other question is, when I write the parametric limits, what does the "Start Value" mean? Is there an extra way to determine it, or is it through trial and error?

The start value is just the value where the algorithm starts when it tries to find the minimum of the objective function. There is no „correct“ way to determine it, usually you either just try a reasonable guess for the parameter value, or you try „randomize start values“ which will choose a different random start value each time you start the fitting. 

And could you please tell me how to use the validation option? Thanks.

The setup of the validation data is very similar to the „experimental data“. The difference is that for the "validation data“ Copasi calculates how good the model fits this data, but Copasi does not actually use this information to improve the fit. The idea is that you use one fraction of your data for fitting (under „experimental data“), and then test if you model also predicts some other date (under „validation data“) that was not used for fitting. 

Sven

<Captura de pantalla 2025-12-18 133908.png> 

This is my result, using Evo programming. with: Number of generations 200 y population size of 20<Captura de pantalla 2025-12-18 134327.png>

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