The glossary is intended to help those of you who are new to the world of sabermetrics and/or those who want a quick definition of certain advanced statistics.



Weighted on-base average (wOBA): Not all hits are created equal like batting average suggests. wOBA attempts to measure a hitter’s overall offensive value by realizing that a single isn’t worth as much as a double and so on.

Weighted Runs Created Plus (wRC+): Basically the same thing as wOBA, except it’s measured in runs, involves park factors and is compared to league average. A hitter with 100 wRC+ is considered average. So, a player with a 110 wRC+ created 10% more runs than average.

Isolated Power (ISO): Measures how often a player hits for extra bases.

Batting Average on Balls in Play (BABIP): Measures how often a ball in play goes for a hit.

Run Expectancy based on the 24 base-out states (RE24): Attempts to quantify how well hitters capitalize on their opportunities. RE24 gives more credit for hits with runners on base than with the bases empty. Baserunners can also improve or diminish their RE24 by advancing on a wild pitch or stealing a base.

Wins Above Replacement (WAR):  Measures a hitter’s total contributions to their team in one statistic. It attempts to explain how much value a team would lose if a player got injured and a minor-leaguer had to replace him.

Hit Probability: Based on the exit velocity and launch angle of the batted ball, how likely a ball is to land for a hit. Expressed as a percentage, it attempts to get to the heart of what a pitcher and hitter control while attempting to take out the effects of defense and ballpark. (Definition courtesy of



Fielding Independent Pitching (FIP): Attempts to estimate a pitcher’s ERA in the future by measuring only what a pitcher can control; walks, hit batters, strikeouts and home runs.

Expected Fielding Independent Pitching (xFIP): Same thing as FIP with one alteration. It replaces a pitcher’s home run total with how many home runs they should have allowed based on the number of fly balls they surrendered with a league average home run to fly ball ratio.

Skill-Interactive ERA (SIERA): Another ERA estimator. SIERA places a higher emphasis on strikeouts, and while walks are bad, they’re not as bad as FIP suggests. SIERA also doesn’t ignore balls in play. SIERA says that pitchers who allow less contact will force weaker contact from hitters.

Run Expectancy based on the 24 base-out states (RE24): With league average set to 0, pitchers with positive numbers are preventing more runs than average given the situations in which they have been placed. (Definition courtesy of FanGraphs).

Wins Above Replacement (WAR):  Measures a pitcher’s total contributions to their team in one statistic. It attempts to explain how much value a team would lose if a player got injured and a minor-leaguer had to replace him.



Defensive Runs Saved (DRS): Total Defensive Runs Saved indicates how many runs a player saved or hurt his team in the field compared to the average player at his position. (Definition courtesy of FanGraphs).

Catch Probability: A number that can be applied to every tracked batted ball to the outfield, and it’s on a scale of 0-100 percent, where a zero percent Catch Probability is “that ball is never, ever caught” and a 100 percent Catch Probability is “caught by everyone, always.” (Definition courtesy of



Steamer: Steamer is the projection system I look at the most. It’s FanGraphs’ go-to system, along with ZiPS, and is considered one of the best projection systems out there.

ZiPS: Developed by Dan Szymborksi of ESPN, ZiPS is also used by FanGraphs and is very well-known among baseball circles.

RW23: The First Out At Third’s own projection system named after Rickie Weeks.


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