The Chicago Cubs are the unquestioned best team in baseball at the moment. There is no aspect of the game where the team struggles. They hit, hit for power, field and run the bases at a high level, pitch well as starters, and pitch well as relievers. When we ask questions and delve into the numbers, we do not ask if they are good. Instead, we ask how good are they, how this happened, and who is responsible. On the hitting side of things, numbers are easier to come by and believe in. On the run-prevention side, however, assigning value between pitching, defense, and luck can be difficult.

Back in June, August Fagerstrom noted that the Cubs’ opponent BABIP, then at .250, was basically the lowest of the past 55 years when adjusted for league average. Back in June, we had not yet completed half the season. Now in September, with the season nearly complete, the Cubs BABIP has risen… all the way to .251, increasing just one measly point. The Cubs are preventing balls in play at a record level.

On balls in play there are three principal groups of actors: pitchers, hitters, and defenders. While an individual hitter might have a decent amount of control over whether a batted ball becomes a hit or an out, pitchers face so many different hitters over the course of a season that, for any one pitcher and any one team, the control by the pitcher and defense on batted balls is likely very influential. So how do we break this down?

First, let’s back up a step, and note something else the Cubs have been doing at a historic level. Generally speaking, a team’s FIP is going to be fairly close to a team’s ERA. Since World War II, there have been 1,716 team seasons, and all but 108 (6.3%) have produced an ERA and FIP within a half-run of each other; two-thirds of teams, within a quarter-run. The Cubs are one of the biggest outliers we have ever seen.

Biggest FIP-Beaters Since World War II Season Team ERA FIP E-F 1954 Giants 3.10 3.86 -0.76 1999 Reds 3.99 4.74 -0.75 1948 Indians 3.22 3.94 -0.72 2016 Cubs 3.08 3.80 -0.72 2002 Braves 3.14 3.83 -0.69 1965 Twins 3.14 3.81 -0.67 1955 Yankees 3.23 3.90 -0.67 1990 Athletics 3.18 3.84 -0.66 1967 White Sox 2.46 3.11 -0.65 1957 Yankees 3.00 3.65 -0.65

So we see the Cubs up there, and wonder what could be causing this. Do the Cubs have a secret sauce? Is it the pitching? Is it the defense? Is this luck?

First, let’s find some statistical causes that would allow a team’s ERA to beat its FIP. Two of the main components for which FIP doesn’t account and that ERA does are (a) sequencing (i.e. the order in which events happen and whether an event occurs with runners on base), and (b) balls in play (i.e. whether a ball is a hit or an out when it stays in the ballpark). The two relevant statistics that measure those things are left-on-base percentage (LOB%) and BABIP. Here’s the chart above, with LOB% and BABIP included.

Biggest FIP-Beaters Since World War II Season Team LOB% BABIP ERA FIP E-F 1954 Giants 77.6 % .255 3.10 3.86 -0.76 1999 Reds 74.2 % .262 3.99 4.74 -0.75 1948 Indians 74.5 % .252 3.22 3.94 -0.72 2016 Cubs 78.1 % .251 3.08 3.80 -0.72 2002 Braves 77.2 % .271 3.14 3.83 -0.69 1965 Twins 76.7 % .250 3.14 3.81 -0.67 1955 Yankees 75.9 % .247 3.23 3.90 -0.67 1990 Athletics 75.7 % .256 3.18 3.84 -0.66 1967 White Sox 76.9 % .245 2.46 3.11 -0.65 1957 Yankees 77.2 % .254 3.00 3.65 -0.65

You might notice that all of those teams conceded pretty low opponent BABIPs, with the highest being the 2002 Braves’ .271 mark. Those LOB% marks are quite high, as well: the league average is generally in the low-70s — this season, it’s 73% — indicating that the Cubs are stranding runners at high levels. Last season, Ben Lindbergh took a pretty in-depth look at a St. Louis Cardinals rotation that had produced a very low ERA and very high LOB%, searching for the causes of that disparity. The Cubs have joined those ranks this season.

Highest Left-on-Base Percentage Since World War II Season Team LOB% 1968 Tigers 79.5 % 2015 Cardinals 79.4 % 1972 Indians 78.8 % 1972 Athletics 78.1 % 2016 Cubs 78.1 %

The Cubs have recorded a historically high LOB% and historically low BABIP. Those two statistics explain most of the difference between FIP and ERA. For the 1289 pitchers who have qualified for the ERA title since 2002, the correlation coefficient between BABIP and ERA-FIP is 0.70 and the correlation coefficient between LOB% and ERA-FIP is -.73. On a team level, in that time, the correlation is even stronger for BABIP (.79) and LOB% (.81). A multiple linear regression for those two stats and ERA-FIP produces an r-squared of 0.80 using the 450 team seasons over the past 15 years.

When Lindbergh looked at the Cardinals last year, he found the Cardinals were getting considerably better results with runners on base — in which situation most teams actually fare worse. The Cubs also have bucked the trend of faring worse with runners on base results-wise: teams have posted nearly identical wOBA (.274 with bases empty, .276 with runners on) despite a higher FIP, thanks in part due to a slightly lower BABIP (.246 with runners on, .255 with bases empty). This is part of the equation when it comes to the Cubs beating their FIP, and this part is going to be mostly luck-based. As that’s not particularly satisfying so far as explanations go, let’s move on to the other part: BABIP.

We already know the Cubs have historically low BABIPs right now, and that’s true both for the starters and the relievers. As the starters get most of the innings, let’s look at their BABIP figures from this year, last year, and their careers overall.

All five starters are posting BABIP numbers well below both their career averages and also last season’s figures. There are certain pitchers who might be able to post low BABIPs regularly, but those pitchers are not only rare but also difficult to identify. Over the last 15 years, 142 starters have logged 1,000 innings and only one pitcher, Chris Young, has posted a BABIP below .265, and he is a very rare, very tall, extreme fly-ball pitcher. Of the 10 pitchers who even posted a BABIP under .275, only Clayton Kershaw and Carlos Zambrano were ground-ball pitchers. It’s possible Arrieta might be in that Kershaw-type mold right now, but the rest of the pitchers on the staff have career numbers that don’t match with what is going on this season.

Even if you ignore career numbers, it’s possible you have heard about pitchers inducing weak contact, and perhaps that is part of the reason why the Cubs’ pitchers are so good. Cubs pitchers are among the leaders for lowest average exit velocity. There’s some evidence to suggest that an average low exit velocity limits home runs, but there is little to no relationship between average exit velocity and BABIP. We know exit velocity plays a strong role for hitting outcomes, especially slugging, but there is little to no relationship with the hard, medium, and soft designations. Weak contact can lead to good numbers for the Cubs’ pitchers. Moreover, most of the Cubs staff have above-average FIPs and have collective produced the fourth-best WAR in baseball. However, being recording strong fielding-independent numbers doesn’t necessarily lead to a low collective BABIP — and that’s a big part of what’s allowing the Cubs pitchers to beat their FIP.

So if pitcher skill isn’t lowering BABIP, how about defense? The Cubs have a fantastic defense, with good to great defensive players all over the field. They upgraded to Jason Heyward from Jorge Soler in right field. They made Addison Russell the full-time shortstop over Starlin Castro and put Ben Zobrist at second base. They have deployed Javier Baez all over the infield, and Kris Bryant has taken to the outfield fairly well in addition to his duties at third base. While single-season UZR numbers come with some sample-size problems, it is interesting to note that the Cubs’ UZR of 67 runs above average confirms the eye test and the individual players’ reputations and is about 25 runs ahead of the second-place Giants this season.

Over the last 15 seasons, 29 teams have posted a UZR of at least 50 runs above average, and the average BABIP against for those teams is just .284. Only two teams, the 2006 and 2007 Kansas City Royals, posted BABIPs above .300 in those seasons. Looking back at the UZR and BABIP over the last 15 seasons, a sample that includes 450 teams, the correlation coefficient between the two is -0.56, fairly strong. The graph below shows all of those teams.

With a few weeks of the season left to go, the Cubs’ UZR is the seventh-highest of the past 15 years, and could go higher before the season is out. In an ideal study, we might regress all of the defensive numbers and get three-season sample sizes, but doing that is still going to confirm that this Cubs defense is a very good group. We might try to avoid taking away credit from pitchers, but the Cubs pitchers are good without giving them extra credit for the work of others. Weak contact and low exit velocities might help make the Cubs good pitchers, especially when it comes to limiting home runs and extra-base hits; however, a lot of pitchers give up weak contact and have low exit velocities, and none of them are beating their FIP quite like the Cubs are. None of them have a defense like the Cubs do, and those players deserve a ton of credit for their share of the work.