Though likely to upset art historians, economist David Galenson's market-based approach to 20th-century art has resulted in what he considers a definitive "best of" list. By analyzing the number of times masterworks appeared in 33 art-history textbooks, Galenson determined that Pablo Picasso's Les Demoiselles d'Avignon was the greatest artwork of the last century. Vladimir Tatlin's Monument to the Third International ranked second, followed by Robert Smithson's Spiral Jetty. In response, John Elderfield, chief curator emeritus at New York's Museum of Modern Art, said that numbers can only explain so much: "There are great, great things being made which are not reducible to statistics."