As of this week, the official start of spring training is only one month away. Since we're getting close to the beginning of Baseball: 2013 Edition, I thought I would give a basic introduction to some of the advanced stats you'll be seeing on this site and around the internet during the season. There's a lot of information to cover, so I'm going to break this into three parts over the rest of this month. We'll begin today with offensive stats.
Batting average on balls in play, or BABIP as it's commonly known, measures what percentage of balls a batter puts in play fall for hits that are not home runs. It's largely driven by luck, but defense and talent play an important role as well. BABIP is calculated by taking a player's hits, subtracting home runs, and dividing it by the number of balls in play:
BABIP = [Balls in Play - HR] / [AB - K - HR - SF]
Typically, 30% of balls in play will fall for hits (it can be slightly higher for quick players). If a player's BABIP is significantly higher or lower than their career average, you can expect it to regress back to the mean in the future. When one of the Blue Jays inevitably gets off to a cold start this season (yes, I am a pessimist, why do you ask?) a good place to look is his BABIP. If it's lower than his career average, there's a good chance his bad numbers are luck driven (though this is not the only factor, obviously). If it's not lower? That's when we have a real problem.
I suggest watching this video for a good explanation of BABIP:
OPS and OPS+
On base plus slugging is a fairly self-explanatory statistic: it's simply the sum of a player's on base percentage (OBP) and slugging percentage (SLG). On base percentage measures the percentage of plate appearances a player gets on base, and slugging measures the average number of bases a batter reaches in a plate appearance. League average OPS varies on a year-to-year basis, but is typically somewhere around .730. The scale is around the same as marks from school: .70-.75 is average, low-to-mid .80s is good, .90s is excellent, and 1.00+ is incredible.
OPS+, on the other hand, adjusts OPS for park effects and is normalized to give 100 as the league average every year. Each point up or down from 100 is 1% above or below league average. So if a player's OPS+ is 125, that player is 25% better than average.
OPS is not without flaws, of course. The biggest one is that it weighs OBP and SLG equally, when it has in fact been proven that OBP is about 1.7 times more important. But calculating and interpreting it is very simple and can be done on the fly.
Weighted on-base average was created by Tom Tango and is used to measure the value a player provides offensively. FanGraphs' Sabermetrics Library post on wOBA describes it quite well:
"Weighted On-Base Average combines all the different aspects of hitting into one metric, weighing each of them in proportion to their actual run value. While batting average, on-base percentage, and slugging percentage fall short in accuracy and scope, wOBA measures and captures offensive value more accurately and comprehensively."
The formula for wOBA changes slightly every year, with the various weights on walks, HBP, singles, doubles, triples, home runs, stolen bases, and caught stealing shifting from year to year (you can find the formulas for wOBA from 1871-2010 here). League average wOBA also changes every season, but is always somewhere around .320.
wRC and wRC+
Weighted Runs Created (wRC) sets out to quantify a player's offensive value. Based off wOBA, it combines a player's offensive stats to find out how many runs a player has provided his team.
You'll most often find it in it's park and league adjusted form, wRC+. wRC+ is similar to OPS+ in that both consistently scales league average to 100, and each point up or down from there is a percentage point above or below average. wRC+ is considered to be more accurate than OPS+ due to the flaws of OPS discussed above. Since wRC+ is based on wOBA, it's a wiser choice when looking to see how a player is contributing offensively. You can find wRC+ on the FanGraphs' statistics pages (Baseball Reference uses OPS+).
K% and BB%
Strikeout and walk rates are quite simple: they're measured by taking the number a times a player has struck out or walked, respectively, and dividing that number by plate appearances.
Like most of the stats we've covered today, league average walk and strikeout rates vary from year to year. Generally speaking, though, a strikeout percentage below 10% and a walk percentage above 15% is excellent, while 18.5% (K) and 8.5% (BB) are average.
In the next couple of weeks we'll take a look at pitching, defensive, and win probability stats. In the meantime, I encourage you to check out FanGraphs' Sabermetrics Library for more information.