BABIP: Batting Average on Balls in Play. This is a measure of the number of batted balls that safely fall in for hits (not including home runs). The exact formula we use is (H-HR)/(AB-K-HR+SF).
Batted-ball statistics: When a batter hits a ball, he hits either a ground ball, fly ball or line drive. The resulting ground ball, fly ball and line drive percentages make up a player’s mix of statistics, with infield fly balls, or pop-ups, being tracked as a percentage of a player’s total number of fly balls.
BB%: Walk rate measures how often a position player walks—or how often a pitcher walks a batter—per plate appearance. It is measured in percentage form.
BB/9: Walks allowed per nine innings
DRS: Defensive Runs Saved. DRS rates players as above or below average based on “runs,” with data from Baseball Info Solutions used as an input. It tracks a number of different aspects of defensive play, including stolen bases, double plays, outfield arms, robbing home runs and range.
ERA: A pitcher’s total number of earned runs allowed divided by his total number of innings pitched, multiplied by nine.
ERA-: A pitching version of wRC+: 100 represents a league-average ERA, and a smaller ERA- is better.
ERA+: ERA measured against the league average and adjusted for ballpark factors. An ERA+ over 100 is better than average, less than 100 is below average.
FIP: Fielding Independent Pitching. A measure of all things for which a pitcher is specifically responsible, the formula is (HR*13+(BB+HBP)*3-K*2)/IP, plus a league-specific factor (usually around 3.2) to round out the number to an equivalent ERA number. FIP helps you understand how well a pitcher pitched, regardless of how well his fielders fielded.
FIP-: A pitching version of OPS+ and wRC+: 100 represents a league-average FIP, and a smaller FIP- is better.
Game Score: A metric devised by Bill James to determine the strength of a pitcher in any particular baseball game. An average Game Score is 50, with the range going from 0 to 114. The highest Game Score ever recorded for a nine-inning game was 105, by Kerry Wood in his famous 20-strikeout game on May 6, 1998. FanGraphs uses a newer version of the metric, called Game Score Version 2.0 (GSv2), which was developed by Tom Tango.
GB%: See batted-ball statistics.
ISO: Isolated power. This is a measure of a hitter’s raw power, or how good he is at hitting for extra bases. Most simply, the formula is SLG-AVG, but you can also calculate it as such: ((2B)+(2*3B)+(3*HR))/AB.
K%: Strikeout rate measures how often a position player strikes out—or how often a pitcher strikes out a batter—per plate appearance. It is measured in percentage form.
K/9: Strikeouts per nine innings
K/BB: Strikeout-to-walk ratio
Linear Weights: The historical average runs scored for each event in a baseball game.
OBP: On-base percentage, an essential tool, measures how frequently a batter reaches base safely. The formula is: (H+BB+HBP)/(AB+BB+HBP+SF).
OPS: On Base plus Slugging Percentage, a crude but quick measure of a batter’s true contribution to his team’s offense. See wOBA for a better approach.
PITCHf/x: Sportvision’s pitch tracking system, which has been installed in every major league stadium since at least the start of the 2007 season. It tracks several aspects of every pitch thrown in a major league game, including velocity, movement, release point, spin and pitch location.
Pythagorean Formula: A formula for converting a team’s run differential into a projected win-loss record. The formula is RS^2/(RS^2+RA^2). Teams’ actual win-loss records tend to mirror their Pythagorean records, and variances usually can be attributed to luck. You can improve the accuracy of the Pythagorean formula by using a different exponent (the 2 in the formula). The best exponent can be calculated this way: (RS/G+RA/G)^.285, where RS/G is Runs Scored per Game and RA/G is Runs Allowed per Game. This is called the PythagoPat formula.
Slash Line: At times, writers may refer to a batter’s “slash line,” or “triple-slash line.” They mean something like this: .287/.345/.443. The numbers between those slashes are the batter’s batting average, on-base percentage and slugging percentage.
Statcast: A tracking technology capable of gathering and displaying previously immeasurable aspects of the game. Statcast collects the data using a series of high-resolution optical cameras along with radar equipment that has been installed in all 30 major league ballparks. The technology tracks the location and movements of the ball and every player on the field at any given time.
UZR: A fielding system similar to Defensive Runs Saved. Both systems calculate a fielder’s range by comparing his plays made in various “vectors” across the baseball diamond to the major league average rate of plays made in those vectors. Both systems also look at other factors such as the effectiveness of outfield throwing, handling bunts and turning double plays.
WAR: Wins Above Replacement. A “win stat” that calculates the number of wins a player contributed to his team above a certain replacement level. WAR is calculated at FanGraphs and Baseball-Reference. Though the two implementations vary a bit, they share a common framework that includes a linear weights approach to runs created, advanced fielding metrics, leverage for relievers and replacement levels that vary by position. In addition, beginning in 2013, both versions unified their definition of replacement level, making the two versions more directly comparable. In this book, it is fair to assume that the version of WAR in question is FanGraphs WAR, or fWAR, unless otherwise noted.
wOBA: A linear weights offensive rating system that is similar to OPS, except that it’s set to the scale of on-base percentage.
WPA: Win Probability Added. WPA is a system in which each player is given credit toward helping his team win, based on play-by-play data and the impact each specific play has on the team’s probability of winning.
wRC+: Like ERA+, wRC+ is scaled so that 100 is average and a higher number is positive. The “RC” stands for Runs Created, but it’s not Bill James’ Runs Created. It’s a “linear weights” version derived from wOBA.
xFIP: Expected Fielding Independent Pitching. This is an experimental stat that adjusts FIP and “normalizes” the home run component according to the number of fly balls a pitcher allowed.
For more information on these and other statistics, visit fangraphs.com/library.