We explore the Mass-Metallicity relation (MZR) for ~1000 nearby galaxies using integrated properties from the extended version of the CALIFA integral field spectroscopy data. We focused on exploring the best mathematical form that describes the observed MZR through different functional forms as well as different statistical environments. To test the goodness of the fit of the MZR, we identify the function that yields the smallest scatter in its residuals. We use this residual to explore possible secondary relations of the MZR with other observables (e.g., SFR, Gas mass, gas fraction, and morphology). Among other results, we note a significant lack of an anti-correlation between these residuals and the SFR, in contrast to previous studies. Our results suggest that the functional form and the presence of secondary relations may depend on statistical treatment.