Since the 1980s, Johns typically produces only four to five paintings a year; some years he produces none. His large-scale paintings are much favored by collectors and because of their rarity are extremely difficult to acquire. His works from the mid to late 1950s, typically viewed as his period of rebellion against abstract expressionism, remain his most sought after. Skate's Art Market Research (Skate Press, Ltd.), a New York-based advisory firm servicing private and institutional investors in the art market, has ranked Jasper Johns as the 30th most valuable artist in the world. The firm's index of the 1,000 most valuable works of art sold at auction—Skate's Top 1000—contains 7 works by Johns.