At the start, please forgive me if my question is too elementary.
I am fitting a non-linear curve. Say a parabola. The data points I have are close to a parabola and the best output I get is a parabola. I want to quantify the quality if fit. Something like an R-Squared metric. I was wondering if the R-Squared metric, like in the case of a linear OLS, makes sense since one of the inputs for R-squared is the average of the input values, which I'm not sure makes sense for a parabola.
Can someone please help?