Rethinking the bullpen with RE24

What is RE24? Some may know the acronym from the prescription drug used in the treatment of edema. Others know it as a tool to evaluate the run expectancy by the 24 base/out states. You probably guessed it, here we are going to talk about the latter.

Introduced by Tom Tango in The Book, run expectancy "is the expected number of runs scored in the remainder of an inning based on the base-out state". And RE24 is nothing else but the difference in run expectancy between the start and the end of a play. For reference, here are the twenty-four base-out states and their run values, thanks to Fangraphs' glossary.

As an example, at the beginning of an inning there is a 0.461 run expectancy. Let's say the lead-off hitter hits a double. The next batter will be in the situation of having a runner on second base (_2_) and no outs, which makes the run expectancy jump to 1.068. So the 2nd batter would get a +.607 (1.068-0.461) RE24 while the pitcher gets penalized with a -.607 RE24.

RE24 is a cumulative statistic, so that during the season, a batter or a pitcher's RE24 will change according to his performance. Therefore, the higher the RE24, the better the player's performance. Thanks to the formidable Fangraphs' glossary, we have a "rule of thumb" to help us asserting an excellent year from a good one etc.

You probably noticed that relief pitchers does not need a RE24 as high as starters to get an "excellent" rating. Considering they get way less time on the mound during a season, this is perfectly understandable. And like many other sabermetric statistics, zero is the average which makes it really easy to distinguish above and below average players.

Now, the reason why I like RE24, especially for measuring the bullpen performance, is because it gives a run-based value for pitchers who have to deal with runners allowed by the pitcher(s) before them. Indeed, RE24 takes that in account thanks to the base-out states.

RE24 can also be a good counter-part to WAR for relievers as they don't have that much options to "capitalize" on wins, although they can do so on runs. Also, when you have a relief pitcher in the sixth, seventh or eight inning, you're thinking about preserving your lead or reducing your gap with the opponent, in terms of runs.

If you're reading Beyond the Box Score (if not you really should), you may have noticed past articles about RE24 and notably this one from last June. Thanks to Ryan Morrison work, you could learn that Dellin Betances, Darren O'Day or Zach Britton were among the best relievers in 2014 in terms of RE24 at the time of the article.

Just for you, I've decided to make my own ranking for the full 2014 season, excluding closers. Why? Because I wanted to pay a closer attention to the other members of the bullpen and closers tend to play mostly in the nine inning where they don't inherit any runner. Thus using RA9 would be as useful as RE24.

Betances is still atop of the league among his peers and in terms of RE24, whilst Kelvin Herrera and Wade Davis made their way to the top three. As for the Rays, Brad Boxberger is in the top 20, proudly standing at the 12th place with a 14.569 RE24 in 64.2 innings pitched. As you can see on the table above, a low FIP does not automatically suggest a high RE24, nor does a high K%. Although, generally a good FIP or K% means that a pitcher have a good probability of having a good RE24 as well, as the graphs below show.

For reference, I used the RE24, K% and FIP of non-closing pitchers who threw more than 10 innings in 2014 as relievers.

So, now that we've gotten more into RE24 and its implications, let's see what this means for the Rays bullpen. Without further ado, here's your ranking of the best Tampa Bay middle and late relievers for 2014:

No surprise at the top as we already saw that Brad Boxberger was the top player of the Rays in 2014, but after him it gets a little bit more messy.

The good news is that three of the top four will stay with Tampa Bay this year.

The other good news is that Kevin Jepsen, one of the new relievers for the Rays, had a 8.88 RE24 in 2014 with the Angels, and that Ernesto Frieri, despite his horrendous performance last year, averaged a 7.01 RE24 in his previous five seasons. If they perform well, they could help the Rays bullpen to crack to the top 10 in terms of total RE24.

Finally, the table below presents the 2014 rankings in terms of Total RE24 accumulated by relievers that are not closers.

RE24 is, in my opinion, an excellent tool to assess of the impact a relief pitcher had on his games. It also helps us to see beyond standard metrics such as walks or strikeouts, even though they are important for pitchers, but not to be limited by WAR as it is more useful for starters due to their amount of innings pitched.

On a team scale, it can also be helpful to assess the ability of a general manager to build a successful team of relievers. Next time, on Serial.

All statistics used in this article come from Baseball-Reference.