David Ortiz has had a spectacular May so far (.351/.397/.754 with 7 HR), pushing back the questions of whether or not the Red Sox would have to cut him. In order to answer whether or not this start was sustainable, I dug into the underlying numbers a bit more. I should probably start with a fairly standard caveat about small sample sizes, particularly with the monthly data. I believe that some of these differences are striking enough to be "real", rather than an artifact due to sample size.
Looking at Ortiz' dismal April and excellent March, there were a few major changes that jumped out to me. To begin with, he cut down on his ground ball percentage from 45.7% to 20%. Ground ball rates are a nice way to look at batted ball data; unlike differentiating between a fly ball and a line drive, there is no qualitative decision when it comes to ground balls. 45% was slightly below average to begin with, but 20% would have been the lowest in the majors in 2009. Cutting down on ground balls is very advantageous for Ortiz for two reasons: first, he is incredibly slow. Last year, Baseball Prospectus "valued" his base -2.4 runs. Second, because Ortiz typically pulls the ball on the ground, the shift kills the effectiveness of ground balls. Unsurprisingly, Ortiz' batting average on ground balls is .125 this season, compared to a league wide average of .250. By eliminating ground balls, Ortiz is getting rid of nearly automatic outs. Keeping his ground ball percentage low would go a long way to maintain his resurgence; while Ortiz has never had a month with so few ground balls, Carlos Pena did only hit a grounder in 29% of his at bats last year, so it isn't unheard of.
The other aspect that Ortiz has improved in May is he is hitting for more power. After having only 7.1% of his fly balls leave the park in March and April, Ortiz currently has a home run per fly ball rate of 30% for May. While this isn't a career high, no one topped 26% in 2009 (Mark Reynolds) and you have to go back to his heyday in the mid-2000's to see any month with similar power production. At this point in his career, I think we can safely say those days are behind us. However, if Ortiz can continue to replace ground balls with fly balls, a regression in the effectiveness of the fly balls may be offset by simply hitting more fly balls. As far as the good news goes, if Ortiz continues to hit 30% ground balls, he should still approach 25 or 30 home runs. If that ground ball rate climbs back up to 40% or 50%, his average and slugging are going to plummet.
As a fun side note, Emilio Bonifacio had the fewest home runs per fly ball in 2009 (1.1%), with one . This home run was hit on opening day, when he also went 4/5 and stole 3 bases. Needless to say, it was all downhill from there.
I'm afraid that the paragraphs above constitute the good news; beyond the successful changes, there are some very scary trends. As a hitter ages, they typically have increased walk and strikeout rates. The general thinking behind this is that as bat speed slows, a hitter becomes more selective. At first glance, David Ortiz seems to confirm this. He's seen 4.57 pitches per plate appearance this year, versus a career high of4.19 coming into this season and his strikeout rates are up to 34.5%, versus a career high of 24.8% last year. Although his strikeout rate for May is lower than in March and April (37% vs 31%, although with only 63 PA in each month, we're looking at a difference of a couple strike outs), it is still higher than any single month during the 2009 season. While Ortiz has seen his strikeout rate increase, his walk rate has actually decreased both in 2010 as a whole, and during his productive May relative to April. Currently, his walk rate for 2010 is the lowest since 2002, his last year with the Twins, perhaps due to a career high in swinging at pitches outside the strike zone. Although the pitches per plate appearance are up, it does not appear to be due to any kind of improved plate discipline.
While Ortiz has been marvelous this May, I believe the underlying numbers are cause for concern. While the change in his batted ball profile make him more likely to be a valuable member of the Red Sox, I believe that his poor plate discipline will eventually catch up with him. Perhaps due to his poor start in 2009 and 2010, more pitchers have been willing to challenge him, allowing for his aggressive approach to pay off. I'm afraid that once the league recognizes the fact that he still has a substantial amount of power, we'll be back to seeing bail out swings on sliders down and away. Hopefully I'm wrong, but if Ortiz becomes unproductive again, things could get very ugly between Ortiz, the team, and the media. In fact, even though he's hitting now, Ortiz is already lashing out at the media, particularly Buster Olney, who questioned Ortiz' ability to get around on an inside fastball.