I am wondering how can I calculate the error of an interpolation (Lagrange, Newton, spline) or approximation (trigonometric) for a function whose mathematical expression I don't know, but I have some points from its representation that I will use to calculate the form of the interpolation / approximation.
I was thinking of an error given by the difference between the:
->mean of the function's values (y's) in the given points that are used to calculate the others
->mean of the function's values (y's) in the calculated points
But this seems to be far from giving any relevant results.