Earlier this week, Casey Boguslaw posted an excellent article over at Baseball Essential regarding Lineup Optimization. The premise of his argument revolved around comparing a team’s wRC+ and their run production per game. In theory, a team with a low wRC+ but high R/G implies that the lineup has been optimized, i.e. they are squeezing every run out of which they are capable out of the lineup. Conversely, a team with a high wRC+ but low run production is suboptimal, and not scoring as much as they should.
Let’s apply this concept to bullpen use. Each team has a certain number of relievers they are able to use in different situations. Similar to the lineup, different points in the game are more or less crucial. This is tracked by the leverage index (pLi). In a few words, a game situation of average leverage has a pLi of 1, with more intense game situations greater than 1, while less intense situations are less than 1. For a bullpen to be optimized, as the leverage increases incrementally, better and better relievers must be used, i.e. the relationship is roughly linear.