where 𝜇 𝑍 (𝜔) = 𝜇 re (𝜔) + 𝑖𝜇 im (𝜔) with 𝜇 re (𝜔) and 𝜇 im (𝜔) the real and imaginary parts of the mean according to (9a), respectively. Conceptually, the OMSE evaluates how well the GP mean matches the exact impedance. The out-of-sample mean square derivative error, OMSE ∂ , is given by