Deadline deals rarely make difference
Most deals you see before July 31 won't alter the pennant races
As the non-waiver trade deadline approaches, there's been a flurry of activity. But how much of a difference do these deadline deals really make? Can a key acquisition make the difference between playing in October and staying home?
We can measure this by looking at a stat called Wins Above Replacement Player, which measures how well a player performs compared to a hypothetical backup. When evaluating in-season moves, this is a good but not perfect proxy for a team's available assets. It's a composite of the options available to a typical team, which can ignore some finer points of roster management. Most teams, after all, make deadline deals to shore up weaknesses, not strengths. So comparison to backups, rather than to an average player, is a better model for the reality most GMs face.
Looking at deadline deals since the introduction of the wild card in 1995, we see a bevy of moves. By rough count, looking at the past-season WARP of the players acquired versus the players given away, we can see that 180 of those deals were ones in which a team was buying talent as opposed to selling, or moves in which little talent was moving in either direction. (In this instance, the buyer is the team that acquired the player with the highest same-season WARP.) But how many of those deals had a significant impact on the division race? Reviewing the record, there are just 10 teams that made "impact" deals.