 One way of alleviating this burden is by constructing approximation models, known as surrogate models, response surface models, "metamodels" or "emulators", that mimic the behavior of the simulation model as closely as possible while being computationally cheap(er) to evaluate.Surrogate models are constructed using a data-driven, bottom-up approach.The exact, inner working of the simulation code is not assumed to be known (or even understood), solely the input-output behavior is important.