In one of my comments yesterday, I pointed out that Brett Wallace has a .380 Batting Average on Balls in Play (BABIP), and that we probably shouldn't be surprised by a regression in his overall batting average. We have already seen a decline in his overall batting average, with a drop below .300 in Tuesday's game with the Rangers. And, given that a .380 BABIP probably can't be sustained in the long run, his batting average may decline further (though it's no certainty).
The mystery for Wallace is that we don't know what the expected mean is when we say that he will "regress to mean." Unlike pitchers, who are expected to allow an average BABIP in the .300 range, the expected BABIP for hitters can vary, based on the hitter's skills. At this point, we don't have a large enough sample for Wallace at the major league level to know what to expect. For that matter, we don't know how Wallace's overall offensive skills will develop at this young stage in his career.
I will use several different approaches to projecting his "true" or expected BABIP.
A good method to compare a player's BABIP is using a "x-BABIP" (expected BABIP) formula. These calculation methods are based on statistical regression analyses that correlate various hitter characteristics to BABIP. Fangraphs provides a link to a "quick calculator" for one such x-BABIP formula, which also recognizes the ballpark of the hitter. The variables which affect BABIP, using this formula, are home runs, strike outs, stolen bases (proxy for speed), line drive percentage, and numbers of flyballs, groundballs, and pop ups.
When I inputted Wallace's combined 2010 and 2011 data, the expected BABIP is .317. This would suggest that Wallace's actual .361 BABIP for 2010-11 is inflated by 50 or more points.
COMPARABLE FIRST BASEMEN
Another approach is to examine the average BABIP for similar hitters. I chose to look at ML first basemen's BABIP for the period 2006 - 2011. The period should be long enough to reduce random variation, and using first basemen (rather than all hitters) should limit the group to hitters roughly similar to Wallace in physical size and speed. In responding to my comment yesterday, OremLK pointed out that Miguel Cabrera's BABIP exceeds .340 despite a lack of speed. That's a good point, but it's also clear that Cabrera seems to be an outlier among first basemen's BABIP. So, I attempted to create two comparison groups to derive average BABIP.
I think line drive rate and groundball rate are logical criteria for developing comparison groups. Both factors directly affect BABIP. Although it is a small sample for Wallace in the big leagues, so far he has shown a high average groundball rate (48%) and line drive rate (20.8%).
The first group of first basemen is based on hitters with groundball rates higher than 40% and line drive rates higher than 18%. These players are arranged in order of ground ball rate below.
2006 - 2011
Player / BABIP
The average BABIP for this group of first basemen with above average groundball and line drive rates is .314.
Maybe we should give more weight to how hard the player hits the ball; and one way of doing that is to set a higher bar for line drive rate (above 20%). I also required that the hitter have a GB/Fly ratio above 1.0. Wallace's average GB/Fly rate in the majors is 1.54. That is very high and likely will decline with a bigger sample. However, as I noted previously, a high groundball rate is directly related to higher BABIP.
2006 - 2011
Player / BABIP / LD Rate
The average BABIP for this group is .316. The very highest line drive rate hitters (Lee, Gonzalez, Cabrera) have a much higher BABIP than the two hitters just above 20% (Delgado, Overbay), but that may not mean anything, given that we are using a sample of five players.
The two comparisons, above, don't do much to detract from the .317 rate produced by the quick calculation, and in fact seem to confirm the x-BABIP. However, if one wanted to be optimistic about Wallace's future, it's not a stretch to see some similarities to Derek Lee in the latter comparison group. Lee maintained a very high line drive rate, exhibited high BB and K rates similar to the 2011 Wallace, shows similar OBP and SLG rates to the 2011 Wallace, and (unlike the more elite Cabrera and Gonzalez) his OPS tends to fall in the mid-800's range, which is the probable range for Wallace too. So, perhaps Lee's .330 average BABIP represents a high side estimate for Wallace's future BABIP level.
WALLACE'S AAA LEVEL BABIP
Wallace has a history of high BABIP rates in the minor leagues. However, those numbers have to be translated to major league numbers, since opposing pitchers and defenders are better in the majors. Dan Symborski at Baseball Think Factory has previously stated that hitters' BABIP declines 4% - 6% in the move from AAA to the major leagues. In approximately 780 at bats, Brett Wallace's average AAA BABIP is .343. Based on an expected decline of 4% - 6%, Wallace might be expected to achieve average BABIP of .323 - .330 in the majors.
WHAT DO WE EXPECT?
There's no set answer to the question. Based upon what I've shown, above, my guesstimate is that Wallace will settle into an average BABIP of .320 or so. Perhaps, at the upper end, Wallace will average a BABIP in the .326 - .330 range, given the potential similarities to hitters like Billy Butler and Derek Lee. But, this is no final word on the matter; feel free to make your own guess.
This also demonstrates the importance of Wallace maintaining the improvement in walk rates that he has shown in 2011. The walk rates will help Wallace maintain a reasonable OBP even if he encounters a BABIP slump. Also, Wallace's ability to improve his power probably means more to his future than his BABIP. Wallace's current ISO (.129) is too low for what we want from a first baseman. However, Wallace's ISO in the minors was much higher (usually above .200); for that reason, ZIPS projects an 11 point increase in Wallace's ISO by the season end. ZIPS pre-season projection for Wallace was a .149 ISO---which happens to be the same as Sean Casey's career ISO. However, if Wallace can achieve an ISO closer to his minor league numbers, then he might have a chance at becoming a Derek Lee type hitter (career ISO of .212).