In this paper, a new methodology to achieve comparative analysis of sags among substations in large distribution networks is proposed. The proposed methodology consists in three stages as follows. First, data acquired from a Power quality measurement system is considered. Second, sags disturbances are classified and quantified according to a proposed level of severity. Third, an index called Sags Activity index-SAI is proposed allowing to weight and rank the severity of sags among the affected substations. The results show that this index might be easily used by both, the electric companies and the regulator entities to establish limits for sags disturbances.