Simple question

Hi guys,

I am quite a newbie in engineering, so apologies for such a simple question.

Essentially, I have a collected data from a process that is a function of two variables, say x and y. When I plot the measured data vs. the two variables, I obtained a surface plot, with the height being the magnitude recorded at (x,y). It is very easy to see that, although very noisy, the surface is essentially flat. I would like to perform some kind of statistical test in order to prove that it is indeed flat, but I just do not know how to do that. Any help would be much appreciated.

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