Can WPA Explain How Teams Buy Relievers?
Over the last decade, much of team behavior has, sooner or later, matched up with new sabermetric discoveries. The Athletics’ “Moneyball” focus on on-base percentage is no longer a secret; teams have begun to quantify the value of defense and spend accordingly; arbitration-eligible players have become more valuable on the trade market. But there is one area in which sabermetric findings quite clearly do not mesh with team behavior, regardless of general managerial regime: spending on relief pitching. At least using Wins Above Replacement, it just doesn’t make much sense. Whereas every non-pitching position on the diamond — as well as starting pitchers — make roughly the same amount per WAR, relief pitchers are on another level, frequently making three to four times more per WAR than other players.
This only doesn’t make sense, however, if we think of teams as buyers of WAR. They aren’t. The teams with the most WAR, although typically in a very, very good spot, will not necessarily win the most games. Teams buy real wins, and the best way we have to measure real wins is with Win Probability Added.