Pound for pound is a term used in combat sports such as boxing or mixed martial arts to describe a fighter's value in relation to fighters of different weight classes. As these fighters do not compete directly, judging the best fighter pound for pound is subjective, and ratings vary. They may be based on a range of criteria including "quality of opposition", factors such as how exciting the fighter is or how famous they are, or be an attempt to determine who would win if all those ranked were the same size. In boxing, the term was historically associated with fighters such as Benny Leonard and Sugar Ray Robinson who were widely considered to be the most skilled fighters of their day, to distinguish them from the generally more popular (and better compensated) heavyweight champions. Since 1990, The Ring magazine has maintained a pound for pound rank of fighters.ESPN.com has a list for mixed martial artists.