From Dan Rosenheck:
In switching Jerry Hairston Jr. for the slumping Nick Swisher, Girardi placed far too much weight on Swisher’s disappointing 38 postseason plate appearances so far — and not nearly enough on the fact that Swisher’s combined on-base and slugging percentages were 159 points higher than Hairston’s in 2009, and 117 points higher over their careers. Girardi chose to sit his powerful right fielder in favor of a player who barely hits enough to play shortstop. Similarly, concerned about Burnett’s supposedly poor synchronization with Jorge Posada, he swapped out his Hall of Fame-caliber catcher for Jose Molina, who has a decent claim to be the worst hitter in the major leagues.
The question of whether “streaks” have any predictive value — whether we should significantly adjust our forecast of a hitter’s performance because he has been “hot” or “cold” in the preceding few weeks — has been studied exhaustively, and analysts are all but unanimous in concluding that we should not.
As Tom Tango writes in “The Book: Playing the Percentages in Baseball,” “Always assume that a player will hit at his projected norm [adjusted for the park, weather, and pitcher he is facing], regardless of how he has performed in the very recent past.”
I did not agree with the decision to sit Swisher, as I felt he was a better bet both offensively and defensively than Hairston even while slumping. However, the reasoning that Rosenheck provides, a favored precept among sabermetricians, seems specious to my admittedly untrained mind. Their studies show that a player in a slump should not be expected to deviate from his career norms in future games. However, just because this idea holds true on a broad level does not mean that a manager should use it when dealing with an individual slumping player.
I would suggest there are two kinds of slumps. The first type is when a hitter is not having any balls fall in, meaning he has not changed his approach at the plate and is not doing anything that would warrant time on the bench. The second type of slump is when the hitter is clearly mechanically off, and could benefit from a day off. If a manager determines that the player is in the second group, does it really make sense to keep him in the game because on a global level, slumps are not predictive of future performance? I would think not. The manager has to address the individual situation, and try to figure out whether this player is doing something clearly anomalous at the plate that is causing his poor performance. In fact, isn’t it possible that the data in the study is skewed by manager actions that help the player out of said slump?
Personally, I thought the fact that Swish was leading the team in pitches per plate appearance in the playoffs meant that he had not changed his approach, and that the hits would therefore come. However, if Girardi felt that he was in the second category of slumping players, a day off probably made sense.