Statistics
OPS+ (Adjusted On-Base Plus Slugging Percentage)
Ever wonder how you could compare hitters across eras, leagues, and/or ballparks in a context-neutral environment? Adjusted OPS, or OPS+, is your answer.
Because of the divergence between run-scoring environments over the decades, between the American League and National League (due to the presence or non-presence of the designated hitter, as well as differing levels of competition) and between the assortment of past and present major league ballparks, the offensive value of a hypothetical .300/.400/.500 hitter playing half his games in Shea Stadium in 1968 obviously would not be commensurate with the offensive value of a hypothetical .300/.400/.500 hitter playing half his games in Rangers Ballpark in Arlington in 1996. OPS+ accounts for this by neutralizing each of these effects and allowing us to juxtapose the performance of hitters whom would otherwise be difficult to compare.
An OPS+ of 100 is always considered league average for a given season, while higher marks are more favorable and lower marks are less favorable. One important consideration is that the additive nature of OPS+ equates to an exaggerated OPS+ in many instances. For example, a hypothetical .270/.330/.430 hitter in a league where the league-average hitter bats .270/.300/.400 would not have a 110 OPS+, despite both his on-base percentage and slugging percentage being 10 percent better than the league average, but rather a 120 OPS+. A brief glance at the basic OPS+ formula reveals why this is:
OPS+ = 100 x [(OBP / league-average OBP) + (SLG / league-average SLG) - 1]
The processes used to calculate the most commonly cited version of OPS+ (via Baseball Reference) are more esoteric, but do properly implement park factors into the equation.
[Additional Reading: "Explaining OPS+ and ERA+" (Mike H., NJ.com)]
wOBA (Weighted On-Base Average)
Developed by noted sabermetrician Tom Tango, wOBA is a linear weights-based offensive formula presented as a rate statistic scaled to on-base percentage. Because wOBA assigns a specific run value to each possible outcome of a given play, the value of each outcome is properly weighted relative to the others (e.g. a home run is worth slightly more than twice as much as a single); Tango's original formula is listed below:
wOBA = (0.72xNIBB + 0.75xHBP + 0.90x1B + 0.92xRBOE + 1.24x2B + 1.56x3B + 1.95xHR) / PA
There are three primary virtues to wOBA: first, it's more accurate than OPS (on-base plus slugging percentage) in that it accounts for the reality that on-base percentage is considerably more valuable than slugging percentage; past estimates have ranged from 50 percent (Bill James) all the way up to 300 percent (Paul DePodesta, "Moneyball"), but today it's generally accepted that on-base percentage is roughly 75-80 percent more valuable than slugging percentage. Second, it's transparent, which is more than can be said for most proprietary statistics that currently proliferate in baseball circles. And third, it's easily translatable to runs, which are the currency of baseball:
Runs Above Average = wOBA - league-average wOBA / (1.15 x PA)
According to Tango himself, "a [league-average] hitter is around .340 or so, a great hitter is .400 or higher, and a poor hitter would be under .300." Career examples: Michael Young (.345), Barry Bonds (.439) and Neifi Perez (.290).
Two commonly cited variants of wOBA exist: FanGraphs' version incorporates basestealing numbers but does not include the RBOE (reached base on error) component of the original formula and is not park-adjusted, while StatCorner's version does not include basestealing numbers, but does include the RBOE component and is park-adjusted.
[Additional Reading: "Weighted On-Base Average" (Tom Tango, Inside The Book), "Another Stat For Your Pleasure ... wOBA" (Rob Neyer, ESPN.com), "The Joy Of wOBA" (Dave Cameron, FanGraphs), "More On-Base Weight Needed For Better OPS" (Rob Neyer, ESPN.com)]