To accurately predict ductile failures of new advanced metals, continuum damage models (CDM's) require experimental determination of material-specific damage parameter(s). While various experimental techniques are being used to determine these damage parameter(s), possible systematic errors due to methodological difference in damage definition have yet to be fully revealed. With the aim of finding the most reliable ductile damage quantification strategy for CDM's, this work provides an in-depth comparison of six theoretically-equivalent methodologies by considering measurement accuracy, precision, damage spectrum, spatial resolution, and practicality. It is found that the methodologies that quantify ductile damage from its geometry introduce significant systematic errors, whereas the methodologies that probe the degradation of a mechanical property suffer from low precision and high complexity, especially for high strains and material anisotropy.