The modern bullpen is still evolving rapidly. I would put it this way: If you compare a typical bullpen of 1938 to a bullpen of 1948, or compare 1948 to 1958, or 1958 to 1968, etc. until today, that in every ten year period there have been obvious changes in the way that bullpens have been staffed and used -- but that never before have those changes been as large as in the last ten years.
These rapid changes are leaving the stat books in their wake. We evaluate relievers by ERA, but a modern reliever can do a lot of damage with runs charged to somebody else. In the 1950s and 60s we developed the concept of the "Save", and since then have added the derivative concepts of "Blown Saves" and "Holds", but the modern bullpen contains one pitcher who is assigned to save the game and six or seven whose job is something else entirely -- something not measured by Saves or anything in their line.
The modern bullpen is staffed by two or three lefties whose job it is to get out lefties, by an eighth-inning guy whose job it is to be a bridge to the closer, by a seventh-inning guy, and by two or three pudknockers whose job is to pitch in where they can. You can have a lot of different guys, doing a lot of different jobs, whose records all look pretty much the same.
We're trying to stretch the record book here to cover more of the modern bullpen. The tables below have 21 categories, not counting the pitcher's name or his team, and rank the best relief pitcher from each team. These are the 21 categories: (All statistics through Sept. 21.)
BS Win: A Blown Save/Win is a "win" credited to a reliever who has blown a save opportunity. Suppose that the starter pitches 7 1/3 innings and leaves with a 3-1 lead, runner on base. The reliever gives up a home run, tying the game 3-3, but the reliever's team scores in their next at bat, making the reliever who has given up the home run the "winning" pitcher. That's a BS Win -- a Blown Save Win.
Holds: A "Hold" is credited to any reliever who inherits a Save Situation, records at least one out, and passes along the Save Situation to the next reliever.
For several years we figured "Save Percentage", which is simply Saves divided by Save Opportunities, and this stat has some currency in the game. But the Save Percentage severely discriminates against middle relievers, who have no real chance to be credited with the Save, since they will be taken out of the game and replaced by the Closer even if they throw 110 miles an hour and strike out everybody they see. Middle relievers typically have Save Percentages of zero, even if they pitch well. The Save/Hold Percentage is a much more realistic evaluation of a pitcher's success in Save situations.