How to compare the fitting of function?

On Tue, 7 Jul 2020 08:57:28 +0200

I would say that the function Gompertz is better than the function
Holling type III; how can I give a number to this hunch?

There are many different goodness-of-fit measures; typically,
regression problems are solved by minimising the sums of squared
residuals, so you can just take a look at those (sum((y.predicted -
y.reference)^2)). Root-mean-square-error [*] is another widely used
metric. When comparing different methods, one should be aware of
multiple comparisons problem [**] and potential for overfitting [***].

All this and more is discussed in books on statistics and regression,
such as Regression Modeling Strategies by Frank E. Harrell, Jr.
[doi:10.1007/978-3-319-19425-7]. For more advice on statistics,
consider dedicated communities such as
<https://stats.stackexchange.com/>, since statistics advice is
considered off-topic here in R-help.

Best regards,
Ivan

[*] https://en.wikipedia.org/wiki/RMSE

[**] https://en.wikipedia.org/wiki/Multiple_testing

[***] https://en.wikipedia.org/wiki/Overfitting