How to fit a curve with an equation

[olibravo]

Exactly, the data don't correspond to photos @CyrilGio uploaded. The k and To coefficients may be easily estimated using lm function, because if we take y = diameter_3d_corrige - pupil_timestamp_secondes^(-0.5) our original problem is just y ~ pupil_timestamp_secondes. For the provided data the estimates are: k = 0.8734 and To = 761.7452. The term pupil_timestamp_secondes^(-0.5) oscilates around 0.03389, so it's very small. That's why the modeled line looks lika a straight line. The problem in Excel was different, indeed, in terms of data. So @CyrilGio, please provide valid data first.
The data don't follow the expected model formula -kt + To + t^(-0.5).

[olibravo]

!?! @CyrilGio , do You understand the difference between sqrt(x) and x^(-0.5) !?!

[CyrilGio]

Hello Olibravo,

Thank you very much for your answer.

I do research so maybe this formula follow datas just in specific cases .. Datas in this post (How to fit a curve with an equation - #5 by CyrilGio) are correct so maybe in this case, this formula doesn't works.

But I don't understand why in this post I can find a curve for this datas : How to fit a curve with an equation - #18 by CyrilGio

And yes about that : sqrt(x) and x^(-0.5) I just understand few days ago, it's the same but the "sqrt" need a "-" and it's the same no ?

Yours sincerely,

GIOVANNANGELI Cyril

[nirgrahamuk]

@CyrilGio

it's the same but the "sqrt" need a "-" and it's the same no ?

no its not the same.

sqrt(4)
4^(0.5)
# both equal 2
4^(-0.5)
# is 0.5