We spend a lot of time talking about value. Which means we spend a lot of time arguing about value, because value is a difficult thing to nail down, given the ambiguity of the word and issues with some of the statistics. We also spend a lot of time talking about future value, which introduces even greater uncertainty on account of the future hasn’t happened yet. Player value is right at the core of FanGraphs, but a lot of the time it’s incredibly complicated. It’s a refreshing break when you can make it easy.

And I don’t know if it gets easier than evaluating relievers, after the fact. It can still be something of a chore, but relative to other players, it’s a breeze. Relievers get inserted in particular places, and they’re supposed to keep the score where it is. A reliever is supposed to do as much as he can to improve his team’s chances of winning. We can see how the performances went by checking WPA. WPA, of course, includes a leverage component, but then, relievers tend to earn their high-leverage responsibilities. Let’s take a brief look back. Let’s talk about some really valuable relievers.

If you’re not familiar, WPA refers to Win Probability Added. At any moment in a game, it’s possible to calculate each team’s chances of winning. It’s an estimate, but it gets close. If you can calculate odds of winning, you can calculate changes in odds of winning. If Pitcher X comes in when his team has a 60% chance of winning, and then when the inning is over it’s a 70% chance of winning, then Pitcher X gets credit for +10% WPA (or +0.10). It’s both a complicated concept and a stupidly easy concept. And it’s interesting to see how the numbers add up over time.

We’ll look at relievers from the last few seasons. You know who the dominant relievers are. Aroldis Chapman is a freak, and you’ve got names like Kenley Jansen, Craig Kimbrel, and Darren O’Day. And so on. WPA isn’t going to make a bad reliever look good. But here are the top 10 relievers over the past three seasons. Odds are you didn’t expect the top two.

In first place, you get the Pirates’ closer, in Mark Melancon. In second place, you get a Pirates setup guy, in Tony Watson. These are the two most valuable relievers of the last three seasons, and while, say, Davis has only relieved two of the years, and Betances barely pitched in 2013, it’s still pretty incredible. Neither Melancon nor Watson counts as a high-profile relief arm, but they’ve done almost everything possible to lead the Pirates to victory. Whatever attention the Pirates get, the Pirates get for Andrew McCutchen. In nerd circles, there’s attention to the front office, and the coaching staff, and Starling Marte. I think even the nerds have underrated the bullpen. This is a huge reason why the Pirates have the second-most wins in baseball over this window, behind only the Cardinals.

Last season alone, Melancon led relievers in WPA, by almost a full win over Betances. Watson finished fifth, nearly tied with Davis. Watson hasn’t been tagged in a FanGraphs post since April, and before that, you have to go to July of 2014. Melancon drew attention to himself last year in the early going because he was missing his normal velocity. Between April 23 and August 2, he allowed two runs. Again, this is hindsight. Maybe you’re a Melancon pessimist. Maybe you think Watson is just all right. We’re not discussing next year. These are two pitchers who’ve been incredibly valuable in years that are already over.

Not coincidentally, over the past three years, the Pirates lead baseball in bullpen WPA, by nearly four wins over the Royals. Last year alone, they led by two wins over the Royals. Last year was particularly terrific. We have this data going back to 1974, so since then, here are the top 10 single-year team bullpen WPA marks:

Best Team Bullpen WPA, 1974 – 2015 Season Team Bullpen WPA 2012 Orioles 13.9 2015 Pirates 11.8 1984 Tigers 11.6 2002 Braves 11.2 1995 Indians 10.7 1982 Red Sox 10.6 2003 Dodgers 10.6 2001 Yankees 10.5 2006 Twins 10.4 2001 Mariners 10.2

Last year’s Pirates rank second, out of 1,168. The 2013 Pirates are tied for 15th. The 2014 Pirates are in the upper fourth. So, actually, are the 2012 Pirates, who I didn’t include. But looking at the data from last year, the Pirates beat their Pythagorean record by six wins, and they beat their BaseRuns record by seven wins. That’s a difficult achievement to repeat, but a big part of how the Pirates were able to do that was quality high-leverage bullpen performance. It wasn’t all Melancon and Watson, but those were the two key cogs, as they’ve been now for a few years. And it wasn’t long ago that Melancon was flipped by the Red Sox, and Watson had trouble against righties. Melancon, in 2012, had an ERA over 6. Watson, in 2012, faced lefties half the time. Watson, in 2015, faced lefties a quarter of the time.

How have the Pirates been able to be so good? They’ve gotten some star-level performances, and they’ve had dozens of quality supporting performances. They’ve squeezed more than you’d expect out of so many players, and Melancon and Watson are among them, interesting arms who’ve blossomed into two of the most valuable relievers in the game. So the Pirates, in turn, have featured one of the most valuable bullpens in the game, locking in scores from the sixth inning on. As Melancon and Watson go, some of it is talent. Some of it is clutch performance, some of it is health, and some of it is strong, smart reliever management. The top of the Pirates bullpen hasn’t been feared like the top of the Royals bullpen. And the Pirates haven’t won the World Series, and Mark Melancon isn’t as good as Wade Davis. But in the regular season, it’s the Pirates who’ve been the best. It’s been quiet, but it’s been undeniable.