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The biggest questions about our list of the most dominant athletes of the past 20 years

This story appears in the 20th anniversary issue of ESPN The Magazine. Subscribe today!

Peter Keating answers all the burning questions we know you're asking about how he compiled The Dominant 20 athletes list.

Where the heck is Michael Phelps?

Here's the issue: There were 34 swimming events at the 2016 Olympics, with three more coming in 2020. And while it makes no sense to count each as a distinct sport, like soccer or baseball, there's also no way to combine times across distances, disciplines, medleys and relays-and no one rating that weights national, international and Olympic competitions for an annual score among individual swimmers. We'd love to tell you where Phelps and Katie Ledecky rank on our list of most dominant athletes, but we can't. It's not our fault, it's the data!

Where the heck is Kelly Slater?

To decide whether to include a sport in our calculations, we used revenues, which account for fan attendance, viewership (through media deals) and reach (through sponsorship). Then we chose $100 million a year as our threshold, because we had to draw a line between the NFL and, say, the Federation of International Lacrosse ($550,000 in 2016), which, it must be said, likely isn't lucrative enough to attract a larger pool of elite competitors. Surfing ($19 million) and action sports generally fall far below that threshold. Slater and Layne Beachley have been dominant, as have snowboarders Shaun White and Kelly Clark, but their sports are on the wrong side of the economic divide.

Where the heck is Kobe Bryant?

Bryant led the NBA in each of two categories six times over his 20-year career: field goals attempted and field goals missed. In 2005-06, when he scored 35.4 ppg, he took 350 more shots than anyone else. Which is to say, he was nowhere near as efficient as his most elite peers. And so while Bryant ranks in the all-time top 20 in value over replacement player, he never led the league in VORP (or win shares). So ... there's that.

Why the heck isn't Serena Williams higher?

You can make a convincing case that at certain moments, Williams has been the most dominant athlete of the past 20 years. In August 2015, the distance in WTA ranking points from Williams to No. 2 Maria Sharapova was as wide as the gap between Sharapova and No. 138 Ons Jabeur! However, as Howard Bryant details in his essay, after Williams first reached the top of the world rankings, she missed significant time for a host of reasons. Serena's dominance is all the more incredible because she has rebuilt it, and more than once. It's just been interrupted.

Why the heck is Peyton Manning so much higher than Tom Brady?

For one thing, today's QB stats are steadily inflating. (There were 1.72 passing touchdowns for every interception in the NFL last season, up from 1.29 20 years ago.) For another, Brady wasn't really Brady-automatic Pro Bowler and MVP candidate-until 2007. Result: Manning considerably outpaces him when measured by yardsticks that don't depend on era, like MVPs (5-3), first-team All-Pro selections (7-3) and number of seasons throwing for 4 percent or more of the league's passing TDs (12-9, adjusted for number of teams). Then there's this: In evaluating players, we considered regular-season stats only, since there's no good way to compare playoffs across sports.

What the h-e-double-hockey-sticks is Barry Bonds doing on this list?

Reliable journalists have reported that Bonds has used androstenedione, Winstrol, Deca-Durabolin, Clomid, trenbolone and BALCO Labs' designer steroid THG. Amphetamines too, but who's counting? Not MLB. Baseball didn't start drug-testing until 2003, never caught Bonds and left his stats untouched in the record books. On the other hand, the international cycling union has wiped numerous records, including Lance Armstrong's seven Tour de France titles. As a result, he barely registered in our calculations. We're judging numbers here, not characters.