It's not the most scholarly thing in the world, but this Chessbase biography of de Groot suggests that where de Groot measured the distinction between novices and experts, Chase and Simon were the first to measure differences between random positions and game-like positions.<p><a href="http://www.chessbase.com/newsdetail.asp?newsid=3290" rel="nofollow">http://www.chessbase.com/newsdetail.asp?newsid=3290</a>