MLB Pythagorean Wins: Can You Believe Your Eyes?

Matt Barnes #32 of the Boston Red Sox. Douglas P. DeFelice/Getty Images/AFP

“I can’t believe my eyes.” That phrase could easily be the motto for the first month of any MLB season. It’s a long season, and small sample sizes in April can’t always be trusted. Can you believe a team or player’s fast start or April slump? Or will things work themselves out to what you expected in the preseason?  

Thankfully, there are plenty of advanced statistics that help you decide whether you can trust an April trend will stand up into summer and beyond. Today, we look at the importance of run differential.  

Pythagorean Theorem 

On April 30 of the last MLB season, there were plenty of surprise teams leading their respective divisions. In the American League East, the Red Sox, expected to struggle, had a 3.5-game lead. In the central, the woeful Royals were up by a game and a half. In the West, the A’s led by a full game.  

The National League didn’t make much sense either. The Phillies were up by a half-game in the East. The Brewers were led by two in the Central, and the very surprising San Francisco Giants were leading the loaded NL West by a half-game.  

Like anything that benefits from a fast launch and soars high, gravity can be expected to eventually pull most of those teams back to earth, but which ones? And which teams will be able to make their fast start hold up? In a proven statistical fact that can be hard to trust on the surface, there’s something more predictive of a team’s success than its actual win-loss record: It’s run differential

It’s called Pythagorean wins, and it was invented by baseball’s analytics pioneer, Bill James. It basically says that looking at how much a team has outscored—or been outscored by—opponents gives you a sense of how many games they “should” win. 

In short, teams with strong offenses and strong pitching, measured by runs scored and allowed, will eventually win more games, even if luck and chance have clouded the results in a small sample size.  

Ready to Fly High 

Out of those early 2021 leaders, which teams had the best run differentials?  

The Red Sox: Plus 24 

The Giants: Plus 23 

The Brewers: Plus 10 

Sure enough, the Giants and Brewers ended up winning their divisions. The Red Sox finished second but advanced to the ALCS before finishing just shy of the World Series.  

Struggles on the Horizon 

Let’s look at the other three division leaders from last April 30: 

The Royals had a run differential of negative two (meaning opponents had scored two more runs against them than they’d scored). Oakland was at negative four. The Phillies were at negative 15.  

Baseball’s version of the Pythagorean theorem was telling us not to trust those teams to continue their fast starts. According to the math, they “should” be teams with losing records, not division leaders, despite opening a combined 44-33, with a .571 winning percentage. 

Over the next five-plus months, the teams proved that Pythagoras knew what he was talking about. Kansas City, Philly, and Oakland combined to go 198-211 the rest of the way for a .484 winning percentage.  

The Phillies were six and a half games out in the NL East and missed the playoffs. The Royals finished fourth in the AL Central, 19 games out. The A’s were nine games back in the AL West and missed the MLB postseason.   

The Astros ended up winning the AL West over Oakland. On April 30, they were in third place, at 14-12, but their run differential of plus 31 showed that better times could be on the horizon. The White Sox won the AL Central, despite being in second place on April 30. Chicago had a plus 29 run differential at the time.  

What Does It All Mean? 

Winning isn’t everything. It’s the only thing. Unless you want to know who’s going to do more winning over the rest of the season. Then you might want to look at how a team is winning. If they’re scoring more runs than their foes, they’re likely to do well. If they’re not, then any early success may end up being a mirage.