Although they may continue to cite them because of their familiarity as reference points, baseball analysts largely have moved on from the historically conventional hallmarks of pitcher and batter performance– ERA and batting average (“BA”), respectively– in favor of more comprehensive metrics that provide a more accurate picture of player performance by addressing some of those traditional statistics’ blind spots.
Focusing here on hitters, some of BA’s most notable blind spots include walks; the fact that each park has different dimensions; and the significant variance in the values of different types of hits (e.g., a single versus a home run). As they have with WAR, the three main baseball-analytics websites each offer their own improved versions of BA: Baseball Prospectus’ True Average (“TAv”); Baseball-Reference’s adjusted on-base-plus-slugging (“OPS+”); and FanGraphs’ Weighed Runs Created Plus (“wRC+”). Visually, TAv looks like a batting average but is scaled every year such that an average hitter has a TAv of .260, while OPS+ and wRC+ are scaled to an average of 100.
If you’ve read baseball articles here or at those websites, then you’ve seen those metrics cited, sometimes seemingly interchangeably, in the course of an examination of hitting performance. As BP’s Rob Mains notes in the first part of a recent two-part series at that site, there’s good reason to treat these three metrics similarly: they all correlate very strongly with each other. (In other words, most batters who are, for example, average according to TAv (i.e., .260) also are average according to OPS+ and wRC+ (i.e., 100).)
There are differences between the three, however, and those differences arise because each regards the elements of batting performance slightly differently. As Rob explained:
How the three derive the numbers themselves, including their respective park factors, is pretty small ball. Bigger ball, though, it what goes into them.
- OPS+ incorporates the same basic statistics as OPS: At-bats, hits, total bases, walks, hit by pitches, and sacrifice flies.
- wRC+ weights singles, doubles, triples, home runs, walks, and HBPs, with the weighting changing from year to year. For example, a home run had a weight of 2.337 in 1968 but only 1.975 in 1996, reflecting the scarcity of runs in the former year. Additionally, wRC+ considers only unintentional walks.
- TAv also weights outcomes, including strikeouts (slightly worse than other outs) and sacrifices (slightly better than other outs). TAv also includes batters reaching base on error and incorporates situational hitting[, which refers to hitting that occurs only when runners are on base: Sacrifice hits, sacrifice flies, and hitting into double and triple plays].
So while all three measures look at the same thing—hitting—they’re not doing it quite the same. For OPS+, a walk is as good as a hit, from an OBP perspective, and a home run is four times as good as a single, per SLG. FanGraphs’ wRC+ weights them, but it doesn’t weight outs, as TAv does. Only TAv considers situational hitting.
When applied to players who are especially good or bad in those areas where the three metrics diverge, the result is a lack of correlation between the three with respect to that player. (Cf. the divergent views of the three WAR metrics with respect to Robbie Ray.) Mains’ second article examines some of those players of whom TAv, OPS+, and wRC+ take different views (e.g., Barry Bonds, Kris Bryant, Ian Kinsler, and David Ortiz) before explaining a few general conclusions:
[TAv, OPS+, and wRC+ are] very similar. You can use any of them and feel confident that you’re usually capturing the key characteristics of a batter.
If you want to drill down, though, here are the differences I found:
The lack of weighting in OPS+ means that it gives slightly less weight to singles and slightly more weight to home runs and walks than TAv and wRC+.
TAv’s inclusion of situational hitting means that batters who are extremely good or bad at avoiding double plays are going to get rewarded or penalized. (Situational hitting also includes bunting, but nobody does that anymore anyway.)
The black box factor in these calculations is park factors. Each of the three sites calculates them their own way. They can account for some changes, though not in a predictable or transparent way like high walk totals or low GIDP rates can.
I expect I’ll continue to use these three metrics somewhat interchangeably in articles at this site, although my preexisting (mostly uneducated) preferences for TAv and wRC+ likely will continue. Articles like Rob’s serve as both an important reminder that, at the edges, these updated metrics aren’t exactly the same and an entry point into thinking more precisely about what we ourselves value in the process of evaluating hitter performance.
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Baseball Notes: Current Issues Roundup
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Baseball Notes: Rule Interpretation Unintentionally Shifts Power to Outfielders?
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Baseball Notes: Preview
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