What we're looking for are the bench players who actually start some games and put up less than 50% of the plate appearances needed to qualify for the batting title.
How does this group hit as a whole? As "lesser players," they might (if we are lucky) give us some sense of where that elusive concept of "replacement value" actually resides.
To look at this in a quick and dirty way, we can query the Play Index to give us various lists of players with less than half of the current (2011) PA total necessary to qualify (not 502 right now, more like 350). Such a list is naturally imprecise and requires some scrubbing, because you have players like Ike Davis showing up on it because they were injured early enough in the season that they appear to be part-timers when they actually are not.
What we get, however, is still an interesting enough set of data for all 30 major league teams. Of course, there is a range of performance, some of which is due to the distortion mentioned above, but is mostly due to the actual distribution of part-time talent.
The percentage of plate appearances given to part-timers in 2011 is around 17%. This contrasts markedly from the percentage of plate appearances given to players who enter the game after it started, which is only about 5%.
We see a marked difference between teams. The range is from the Yankees at the lowest (just over 300 PA thus far) to the Padres at the highest (over 1100 PA).
One reason for measuring this right now is that the effects of mid-season "tactical trading" (just starting to hit as the deadline looms later today) will make accounting for all that much more problematic and time-consuming. (Hint to Forman: here's yet another type of stat breakout to put on the endless enhancement list.)
There's no sense that a team with bad part-time hitting is doomed to a losing season. The teams with the very worst part-time offense (Brewers and Angels) are still in the pennant race, though one would expect that having such poor bench performance isn't going to enhance their chances.
|Ike Davis: not a part-timer, dammit!|
Using those two data points (.629 part time, .746 full time), we see that part-timers hit about 16% worse than full-timers. That's a value that ought to be able to enter into the ongoing discussion of the eternally murky concept of "replacement value." One question that needs to be examined before plugging this value into any discussion of that concept, however, is whether that value has differed in the past, when the offensive bench was larger and platooning strategies were more prevalent. It could be higher or lower as a result of how true platoon players get defined into the data set--if they have enough PAs to get over the "50% of 502 PA", then they aren't technically "part-time" and the data set for the players we've examined here would be smaller (and presumably a good bit more offensively feeble).
We'll revisit this idea later on when time permits. The data exists, however, to measure this breakout over the long haul of baseball history, and there is some potential within it (if it is handled properly) to create a more comprehensive idea of what the basis for "replacement level" ought to be.