Here at RC+, you’ll see us using a variety of non-traditional baseball stats. We understand not every reader will know what these analytics are, and that’s OK! We’re here to help with this primer on some of the most common stats you’ll find in our articles.

**BABIP** (Batting Average On Balls In Play) — A player’s batting average on non-home run batted balls. Because a player — especially the pitcher — has little control over what happens on batted balls, BABIP is often used to show how much luck has factored into their performance. While other factors are involved, players who stray far from their career BABIP may be experiencing a significant amount of good or bad luck.

**Barrels** — A batted ball with the “perfect” combination of exit velocity and launch angle. It results in an expected batting average of at least .500 and an expected slugging percentage of at least 1.500. The batted ball must be hit at least 98 mph to qualify.

**DRS** (Defense Runs Saved) — A defensive metric created by The Fielding Bible that attempts to measure how many runs a fielder saves or gives up compared to the average fielder at their position. League average is zero; a player above that is considered a good fielder, while a player below that rates poorly. Similar to other defensive metrics, it gives players credit (or takes it away) based on the likelihood of making a particular play. A player who records an out that 75% of other players would also make, he gets 0.25 “points” in his favor. If he fails to make the play, he loses 0.75. We need about one to three years of data before making any conclusions about a fielder’s capabilities as measured by DRS. It differs from UZR because it uses a batted ball timer to calculate the probability of making a play.

**Exit Velocity** — The speed at which the ball comes off the hitter’s bat.

**FB%** (Fly Ball Rate) — The percentage of a player’s batted balls that were classified as fly balls.

**FIP** (Fielding Independent Pitching) — An ERA estimator that attempts to isolate a pitcher’s performance only to what he can control: strikeouts, walks, hit batters, and home runs. The metric takes fielding and batted ball luck, which can significantly help or hurt a pitcher, out of the equation.

**GB%** (Ground Ball Rate) — The percentage of a player’s batted balls that were classified as ground balls.

**Hard-Hit%** — The rate of hard contact per batted ball. The measurement varies between tracking systems. FanGraphs uses Baseball Info Solutions to measure the stat (called Hard% on the site) and does not reveal what constitutes “hard” contact. Statcast measures Hard-Hit% as any batted ball with an exit velocity of 95+ mph.

**HR/FB** (Home Run to Fly Ball Ratio) — The percentage of fly balls that go for home runs. It is typically used in pitcher evaluation. Pitchers do have some degree of control over the types of batted balls they allow, but they have very little control over how far a fly ball goes and no control over how big the ballpark is. Although there are exceptions, most pitchers will hover around the league average (15% in 2019). A pitcher who deviates dramatically from the league average or his career norms is likely having unsustainable good or bad luck and will likely regress to the mean.

**IFFB%** (Infield Fly Ball Rate) — The percentage of a player’s fly balls (not total batted balls) that were hit in the infield.

**ISO** (Isolated Power) — A measure of a player’s “true” power, ISO is slugging percentage minus batting average.

**Launch Angle** — The angle at which a batted ball left the hitter’s bat.

**LD%** (Line Drive Rate) — The percentage of a player’s batted balls that were classified as line drives.

**OAA** (Outs Above Average) — A Statcast metric for outfield defense that measures how many outs the player converts compared to the average outfielder. An outfielder who makes a catch on a ball with an 65% catch probability gets .35 “points” toward their season total, whereas an outfielder who fails to make the catch would lose .65 points.

**O-Swing%** (Out-of-Strike-Zone Swing Rate or Chase Rate) — The percentage of balls outside the strike zone that a hitter swung at.

**Pop Time** — The amount of time it takes a catcher to throw the ball to a base on a steal attempt. It is measured from the “pop” of the catcher’s glove to the “pop” of the fielder’s glove.

**SIERA** (Skill Interactive ERA) — The most detailed ERA estimator, SIERA differs from FIP and xFIP in that it does not ignore batted balls. It considers that not all batted balls are created equal. Ground balls, fly balls, and pop-ups are easier to convert into outs than line drives, and SIERA accounts for that in its ERA estimate. Additionally, it gives extra credit to pitchers with high strikeout rates because they’re more likely to induce weak contact. It also doesn’t punish a pitcher with good control for a walk as much as a pitcher with a high BB% because they’re less likely to get hurt by allowing even further baserunners; a high-walk pitcher, however, is more likely to compound the problem by allowing more baserunners.

**Spin Rate** — A pitch’s revolutions per minute. What constitutes “good” spin depends on the pitch, and high/low spin don’t necessarily correlate to a good/bad pitch. In general, high spin is desirable for four-seam fastballs and breaking pitches, while lower spin is better for sinkers, changeups, and splitters.

**Sprint Speed** — Statcast’s measure of how many feet per second a player runs on “maximum effort” plays (weakly hit ground balls that a player tries to beat out for a hit and plays where the runner takes at least two bases).

**SwStr%** (Swinging Strike Rate) — The percentage of swings and misses on all pitches seen or thrown.

**UZR** (Ultimate Zone Rating) — Another defensive metric, created by Baseball Info Solutions, that compares how many runs a fielder saved or gave up compared to the average player at their position. It factors in a player’s range, throwing arm, ability to turn double plays, and ability to avoid errors. It is also park-adjusted. Similar to other defensive metrics, it gives players credit (or takes it away) based on the likelihood of making a particular play. A player who records an out that 75% of players would also make, he gets 0.25 “points” in his favor. We need about one to three years of data to start making any conclusions about a fielder’s capabilities as measured by UZR.

**WAR** (Wins Above Replacement) — A cumulative stat that attempts to measure the number of wins a player adds over a replacement-level player. It incorporates all aspects of a player’s game — hitting, pitching, defense, and baserunning — to estimate their total value. FanGraphs and Baseball Reference both have their own separate equations for WAR.

**Whiff Rate **— The percentage of swings and misses on all swings.

**wOBA** (Weighted On-Base Average) — Rather than treating every outcome the same — as batting average and on-base percentage do — wOBA gives the hitter appropriate credit for the additional value that an extra-base hit will provide. It is set on the same scale as on-base percentage and accounts for hits, walks, and hit by pitches. A .320 wOBA is roughly league average.

**WPA** (Win Probability Added) — Unlike other stats, WPA considers the context of a game situation and weighs the outcome of a plate appearance appropriately. It’s a measure of how much a player adds to or subtracts from his team’s chances of winning. A player who comes through in a high-leverage situation (e.g., a batter getting a go-ahead RBI hit or a pitcher striking out a batter with the bases loaded in a tie game) will earn more WPA than a player who hits a solo home run or records a strikeout in an 8-0 game. On the same note, a player who fails in a high-leverage situation will lose more WPA. Players earn WPA throughout the season; however, you should take caution when evaluating a player with this stat. A player with a high WPA has not necessarily been more “clutch” than someone with a low WPA. It may simply mean they’ve had more chances in high-leverage situations.

**wRC+ **(Weighted Runs Created Plus) — A stat that attempts to measure a player’s total offensive contributions with one round number. League average is 100. Any point above or below that mark indicates the batter has performed X% better or worse than league average. For example, a player with a 145 wRC+ has performed 45% than an average hitter. Like wOBA, wRC+ does not treat every outcome the same; extra-base hits are more valuable than walks and singles. wRC+ is adjusted for parks and the current run environment, which makes it a valuable tool for comparing players across different seasons.

**xBA **(Expected Batting Average) — A useful metric when looking at a single batted ball as well as season-long performance for both hitters and pitchers. xBA estimates what the outcome of a batted ball (or series of batted balls) should have been based on the actual outcomes of previous batted balls with the similar exit velocities and launch angles. On weakly hit ground balls, xBA also factors in a player’s average sprint speed.

**xFIP **(Expected Fielding Independent Pitching) — An ERA estimator that attempts to isolate a pitcher’s performance only to what he can control: strikeouts, walks, hit batters, and fly balls allowed. The metric takes fielding and batted ball luck, which can significantly help or hurt a pitcher, out of the equation. Unlike FIP, xFIP does not use raw home run totals in its calculation. Instead, it estimates how many home runs a pitcher should’ve given up based on their fly ball rate and the league average HR/FB.

**xSLG** (Expected Slugging Percentage) — Like xBA, xSLG estimates what a player’s slugging percentage should be based on the actual outcomes of previous batted balls with similar exit velocities and launch angles. That is, it estimates the probability of the ball going for a single, double, triple, and home run to calculate an expected slugging percentage. Sprint speed is also incorporated on weakly hit ground balls.

**xwOBA** (Expected Weighted On-Base Average) — Another stat that attempts to measure a player’s true performance by evaluating their quality of contact. It mixes this with true outcomes — strikeouts, walks, and times hit by a pitch — to give a nice blend of a player’s performance. Because it incorporates strikeouts, walks, and HBP, xwOBA is particularly helpful for evaluating both hitters and pitchers.

**Z-Swing%** — Percentage of swings on pitches in the strike zone.