Dr. Porter wrote "The Competitive Advantage of Nations" in 1990. The book is based on studies of ten nations and argues that a key to national wealth and advantage was the productivity of firms and workers collectively, and that the national and regional environment supports that productivity. He proposed the "diamond" framework, a mutually-reinforcing system of four factors that determine national advantage: factor conditions; demand conditions; related or supporting industries; and firm strategy, structure and rivalry. Information, incentives, and infrastructure were also key to that productivity. During April 2014, Porter discussed how the United States ranks relative to other countries on a comprehensive scorecard called "The Social Progress Index", an effort which he co-authored. This scorecard rated the U.S. on a comprehensive set of metrics; overall, the U.S. placed 16th.