The Social Progress Index (SPI) measures social and environmental outcomes in countries across three dimensions - basic human needs, foundations of well-being, and opportunity to progress. It uses 54 indicators to assess relative performance and provide a more holistic view of societal development than economic factors alone. The SPI was created as a complement to GDP because GDP provides an incomplete picture of human development. It follows specific design principles and frameworks to measure and rank countries based on their social progress performance.