Stats Glossary

BABIP – Batting average on Balls in Play (BABIP) is a statistic that measures how many batted balls in play land for hits. Home runs are excluded and the formula for BABIP is (H-HR)/(AB-K-HR+SF). League average BABIP is generally around .300, but hitters have shown they can affect their BABIP more than pitchers. BABIP is generally used to spot ‘fluky’ performances. Read more about BABIP, here.

Barrel Rate – batted ball events whose comparable hit types have led to a minimum .500 batting average and 1.500 slugging percentage.

BB% – Walk percentage. The percentage of plate appearances that result in a walk. BB% is (BB/PA). The league average BB% tends to be around 8% for hitters and pitchers. Because of the DH, BB% tends to be slightly higher in the AL.

BB/9 – Walks per 9 innings. BB/9 is (BB/IP)*9)). For starters, the league average BB/9 in 2008 was 3.1 walks. For relievers it was 3.9 walks.

ERA+ – Adjusted ERA. Much like OPS+, ERA+ is how well a pitcher performed relative to the league average ERA with ballpark and league adjustments applied. A score of 100 is average. You can find ERA+ readily available on Baseball-Reference.

Exit Velocity – measures the speed of the baseball as it comes off the bat, immediately after a batter makes contact.

FB% – The percentage of balls hit into play that are classified as flyballs. For pitchers, league average FB% tends to be around 36%.

FIP – Fielding Independent Pitching. FIP examines a pitcher’s core responsibilities: home runs allowed, strikeouts, and walks. Because the quality of a defense that plays behind a pitcher can wildy vary, FIP removes defense altogether. FIP is scaled the same way that ERA is.

According to FanGraphs: “The formula for FIP is: (HR*13+(BB+HBP-IBB)*3-K*2)/IP, plus a league-specific factor that scales FIP to match league average ERA for a given season and league.”

GB% – The percentage of balls hit into play that are classified as groundballs. For pitchers, anything above 55% is very good. For example: Brandon Webb has a career GB% of 64.2

ISO – Isolated-Power. A measure for how much “true” power a player hit for. Because batting average treats every hit as a single, ISO removes BA from SLG to give you an idea of how much power a player hit for. The formula for ISO is (SLG-AVG). League average ISO tends to be around .150. For example: Barry Bonds’ career ISO is .309

K% – Strikeout percentage. The percentage of at-bats that result in a strikeout. K% is (SO/AB). The league average K% tends to be around 16% for starting pitchers and slightly higher for relief pitchers.

K/9 – Strikeouts per 9 innings. K/9 is (SO/IP)*9)). For starters, the league average K/9 in 2008 was 6.5 strikeouts. For relievers, it was 7.5 strikeouts.

Launch Angle – the vertical angle at which the ball leaves a player’s bat after being struck. Average Launch Angle (aLA) is calculated by dividing the sum of all Launch Angles by all Batted Ball Events. As a guideline, here are the Launch Angles for different types of contact (MLB.com):

Ground ball: Less than 10 degrees
Line drive: 10-25 degrees
Fly ball: 25-50 degrees
Pop up: Greater than 50 degrees

LHP – Left-handed pitcher.

LOOGY – Lefty One-Out Guy. A pitcher who’s primary duty is to get left-handed batters out. Often called a “lefty specialist”.

OPS – A quick and dirty offensive statistic. OPS stands for on-base plus slugging or (OBP + SLG = OPS). Generally, an OPS of .800 or greater is considered to be good. In 2008, the average OPS for the MLB was .749. There are much better stats to use besides OPS these days, but it’s not bad stat to quickly gauge a player’s offensive ability.

OPS+ – Adjusted OPS. OPS+ is OPS adjusted for league and park. It’s based on the scale of 100 = an average offensive player. Anything below 100 indicates that the player was a below average hitter (regardless of position, it’s just looking at hitting) and anything above 100 indicates a player was an above average hitter. Like OPS, it’s a quick and easy way to gauge a player’s offensive worth. It’s preferable to OPS since a park and league adjustment is included. Each number above (or below) 100 is a percentage point. So, for example, if a player had an OPS+ of 125 it would mean that he’s 25% better than your league average hitter. You can find OPS+ readily available on Baseball-Reference.

PA – Plate appearance. A PA can be considered a player’s turn at batting and is defined as: “PA = AB (at-bats) + BB (walks) + HBP (hit by pitches) + SH (sacrifice hits) + SF (sacrifice flies) + Times Reached on Defensive Interference”.

PFX – PitchF/X is a camera system that first appeared in the 2006 playoffs. By the end of 2007, every park had the system installed. PFX measures every pitch thrown by every pitcher in each season. The system tracks such things as: velocity, pitch-type(s), horizontal movement, vertical movement, spin rotation, at-bat results, and much more.  Learn more about PitchF/x, here. An excellent primer by Mike Fast can be read, here.

RHP – Right-handed pitcher.

tRA – Consider tRA like FIP but with linear weights based on batted ball types. StatCorner, the website that publishes tRA, defines tRA as:

Developed by Graham MacAree, the basic introduction to tRA is located here. tRA involves assigning run and out values to all events under a pitcher’s control and coming up with an expected number of runs allowed and outs generated in a defense and park neutral environment. tRA is on a R/9 scale and does not involve any regression of the rates so while it should be more useful at determining a pitcher’s true talent level, the best method for pitching projection is to use tRA*, the regressed version of tRA.

tRA is another helpful tool to use when trying to determine a pitcher’s true performance without defense muddying the picture.

UZR – Ultimate Zone Rating. Up-to-date data can be found on the excellent FanGraphs website. FanGraphs’ definition of UZR is:

“The number of runs above or below average a fielder is in both range runs, outfield arm runs, double play runs and error runs combined.”

When using UZR data, ideally you’d like to have a sample of 2,000 innings or so before you start trying to peg a player’s true talent level on defense. Most defenders range from -10 runs to +10 runs on defense.

WAR – Wins Above Replacement. How many “wins” (or runs) a player added over the theoretical replacement level player. Replacement level, for our purposes, is defined as freely available talent — ie: talent that’s cheap and easy to acquire. A player that’s worth +2 wins above replacement is an average player. FanGraphs has a good breakdown on WAR here and Tom Tango has a good page on the process, here.

wOBA – Another Tango invention, wOBA stands for weighted on-base percentage. wOBA is an offense metric based on linear weights (every baseball outcome has a specific run value) and is scaled like on-base percentage. League average wOBA tends to be around .330. Below .300 is a poor hitter and above .400 is a very good hitter.

The formula for wOBA is: (0.72xNIBB + 0.75xHBP + 0.90x1B + 0.92xRBOE + 1.24x2B + 1.56x3B + 1.95xHR) / PA

wOBA is one of the top ways to measure a player’s offensive worth. For example: Barry Bonds has a career wOBA of .439 vs. Neifi Perez’s career wOBA of .290. Read more about wOBA, here.

wRC+ – Another  FanGraphs’ statistic. wRC+ is essentially the “wOBA based version of OPS+”. OPS+ is a very handy statistic, but because of the way it’s calculated, it can undervalue guys who take a lot of walks and guys that are good at stealing bases. If you’re looking for a quick statistic to compare hitters across parks and eras, wRC+ provides a fantastic option. Much like OPS+, wRC+ is presented on a scale of 100 being league average. Another note, anytime you see a stat with a “+” on the end, it means that it’s been adjusted for league and park.

xERA – Expected ERA uses exit velocity, launch angle, and sprint speed on “topped” or “weakly hit” balls to model what a player’s ERA would look like based on the underlying characteristics of the balls in play. The best way to use xERA is to compare it to the actual ERA.

For more information on advanced statistics, head over to the Fangraphs Library.