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"Pound for pound" rankings were developed by boxing writers during the era of Sugar Ray Robinson (pictured) to rank the world's greatest fighters irrespective of their weight division.

The nature of these rankings is subjective and raises an interesting question: How do you compensate for differences in size, power and historical time periods when evaluating boxers?

One common approach is to simply assume all boxers were the same size and evaluate them based on that criteria. However, I think that approach undervalues heavier fighters.

For one, it doesn't adjust for the natural variations that come with larger body size—it's impossible to assume that a 5'6" boxer would be physically identical if he were a foot taller. Secondly, there are physical restraints: Even if a heavyweight's and flyweight's fists are moving at the same velocity, the longer arm length of the heavyweight makes the punch look slower and less "snappy."

Alternatively, this list evaluates fighters based on two major factors:

1. Their skill and accomplishments relative to others in their division(s).

2. Their ability to win in multiple weight classes, if applicable.

In developing this list, I examined films and clips of over 250 fights, historical boxing records, anecdotal accounts and other "greatest ever" lists, including those compiled by Ring magazine, ESPN and Bert Sugar. No list can settle the great debate, but this one is the result of hundreds of hours of research and evaluation and is a good starting point.

Without further ado, the 100 Greatest Pound for Pound Boxers of All Time.