(Photo by Jonathan Newton/The Washington Post)

A number of reasons have been floated for why the Washington Nationals are playing better baseball in 2014 than they did in 2013. Clubhouse chemistry, the emergence of Tanner Roark, trades for impact players and the resurgence of Denard Span have all been identified as key motivations behind an improved Nationals club. However, one of the most overlooked reasons has been the improvement of the Nats’ bullpen.

We all know the basic rankings; the Nationals bullpen is third in the National League in ERA and first in MLB in Fielding Independent Pitching, which measures what a player’s ERA would look like over a given period of time if the pitcher were to have experienced league average results on balls in play and league average timing. Not many though have taken note of the difference between those numbers between 2013 and 2014. In 2013, the Nats bullpen had an ERA of 3.56 and a FIP of 3.50, in 2014 those numbers are 2.93 and 3.03, respectively.

In FanGraphs’ FIP based WAR and runs against based RA9-WAR (wins above replacement calculated with RA9. This is both park adjusted and league adjusted) we can get an idea of the impact that basic difference has had. In 2013 the Nationals bullpen was worth a collective 3.0 wins above replacement by the FIP based WAR and 3.1 wins above replacement by RA9-WAR. Through just 137 games in 2014 the bullpen has a WAR of 4.5 wins and a RA9-WAR of 4.8 wins, a full 1.5 win difference. While it is possible for those numbers to go down by the end of the season, it’s unlikely, and it’s even more unlikely that they won’t be an improvement on 2013.

There is a problem though, WAR is designed to remove context to more accurately compare players across positions and teams. This is great, but for a position that relies so heavily on the context under which they enter and exit the game it probably isn’t the best measure. To rectify that, we can look at context-dependent stats, which will give the relievers more accurate credit for the high leverage situations they often pitch in.

The first one is RE24, which is the change in run expectancy based on the 24 base-out states as defined by Tom Tango. RE24 measures an individual player’s impact on their team’s expectation of scoring a run, or in the case of pitchers reducing their opponent’s chance of scoring a run. In 2013, Nationals relievers combined for -2.54 RE24, while in 2014 they have a combined RE24 of 34.31. Considering the rule of thumb that about 10 runs is worth one win, we see a three win difference between 2013 and 2014. At the very least the bullpen isn’t improving Nats opponents’ chances of scoring a run in 2014.

The logical progression from RE24 is win probability added which uses Tom Tango’s win expectancy charts to directly measure how much a player contributes to his team’s chances of winning a game. It takes into account both positive and negative plays to produce one number. In 2013, Nationals relievers had a combined a win probability added of 1.40, while in 2014 they have a combined WPA of 4.64. Since one WPA is equal to 100 percent win expectancy the 2014 bullpen has already been worth three extra wins, the same conclusion derived from RE24.

But what if we don’t want a completely context neutral statistic like WAR, but we also don’t want a completely context dependent statistic like WPA? That’s where perhaps the best statistic for judging relief pitchers is context neutral wins, or WPA/LI. This is a combination of the aforementioned WPA and leverage index which is a measure of how important a game situation is depending on the same factors as WPA. By dividing the win probability by the leverage index, we can directly measure a player’s contributions to his team’s win expectancy, while neutralizing for leverage index so that players don’t get a large bonus by being in high leverage situations more often.

In 2013, the Nationals bullpen had a WPA/LI of 0.41, while in 2014 that number is 3.43. Again, we see the same three win difference as seen with RE24 and WPA. While the exact difference between the 2013 and 2014 bullpen might not be three wins, there is still a stark difference between the two seasons. The 2013 bullpen barely contributed to the Nationals’ win expectancy and actually detracted from its run expectancy, while the 2014 bullpen has put up numbers that rank eighth in MLB in RE24, ninth in WPA and eighth in WPA/LI.

While the overall difference between the 2013 Nationals and 2014 Nationals won’t be just three wins, it certainly is nothing to scoff at. And when we take a quick look at the 2014 and 2012 bullpens in the same statistics presented here they compare favorably if not better than that 98-win squad.

Season Team ERA   FIP   WAR   RA9-WAR    RE24   WPA WPA/LI
2014 Nationals 2.93 3.03    4.5 4.8 34.31 4.64 3.43
2012 Nationals 3.23 3.70    3.5 6.3 36.27 3.32 1.99

While the starting pitchers and position players garner all the headlines, the bullpen is an underrated part of a team’s success. Especially in the low run scoring environment of the 2010’s a bullpen that can both hold wins and keep games close when their team is trailing can have a large impact on a team’s fortune. For the Nationals, not much has been made of their bullpen in the past couple seasons, but perhaps it should have been. The Nats have been at their best when their bullpen is at its best and in 2014 they might have one of the best bullpens in the game.

James O’Hara blogs about the Washington Nationals at Citizens of Natstown. You can follow him on Twitter at @nextyeardc