Cubs defense helping to push for run prevention record

It’s not exactly a hot take to say that the Cubs are the best team in baseball, because they’re 40 games over .500 and their magic number is down to five. They’ve got the lowest staff ERA in baseball. Chicago has scored the third-most runs per game, behind only Boston and Colorado. The Cubs have the most Defensive Runs Saved. They have baseball’s highest walk rate, and they rank well on the bases, too. They’re the most complete team we’ve seen in years.

So as we wait to see if the Cubs can back up all that hype in the playoffs, let’s take a moment to point out that in one important area, they’re not just “pretty good,” they’re potentially “historically good” — at least so far as run prevention goes.

After beating the Astros, 9-5, on Sunday night, the Cubs have a 3.09 team ERA, the best in baseball. That’s nearly half a run better than the second-place Nationals, who sit at 3.43, but perhaps on its own, that 3.09 number doesn’t sound all that impressive; after all, it’s just the fourth-best mark of the 21st century. The 2011 Phillies of Cole Hamels, Cliff Lee and Roy Halladay, for example, had a 3.02 mark.

But because of the way the run environment fluctuates from year to year, looking at raw ERA is less useful than looking at the difference above or below the league average. In the same way that hitting 40 homers would have been far more impressive in 1968’s “Year of the Pitcher” (when only one hitter did so, Frank Howard) than in the hitter-friendly 2000 season (when 16 hitters hit 40), a team’s ERA is only important when compared to what everyone else did that season. When those ’11 Phillies had a 3.02 ERA, the National League had a 3.94 ERA overall. So far this year, with offense up across baseball, that NL number is 4.16.

So rather than just look at ERA, let’s look at an advanced stat called “ERA-,” which is a lot less scary than it sounds. It’s merely “percentage points better than the league average,” and we’ll keep it simple by showing exactly that. Those 2011 Phillies, once park factors were accounted for, had an ERA 21 percentage points better than the NL average. This year’s Cubs, despite having a slightly higher ERA than those Phils, are 25 percentage points better than the league average. That’s how this works.

If we do that over the past 100 seasons of baseball, which comprises 2,180 team seasons, we can come up with a leaderboard. Right now, the Cubs are tied for the fourth-best run prevention season of the past century — and entering Sunday’s game, they were tied for best, before late homers by Yulieski Gurriel and Evan Gattis pushed them back down to second.

1917-2016, best team ERA compared to league ERA

26 percentage …

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