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Because we like baseball, we like statistics. Baseball statistics, that is. You can thoroughly enjoy baseball without paying any attention to its statistics, of course, but to really understand the game deeply, you’ve got to dive in.

Following is a definition of most of the stats we use. You can find most of these stats at the two best baseball statistics websites: Fangraphs and Baseball Reference. By the way, when you see a writer refer to a batter’s performance in this way -- .275/.338/.425 (insert your own numbers) – he’s referring to the batter’s BA/OBP/SLG. If you’re not sure what those are, read on.

A (Link)

Assists. The number of times a fielder touched or threw a ball that resulted in an out (or would have resulted in an out but for an error by the fielder receiving the ball).

AB (Link)

At Bats

BA (Link)

Batting Average, Hits divided by At Bats.

BA/RISP (Link)

Batting Average with Runners in Scoring Position (second and/or third base).

BABIP (Link)

Batting Average on Balls in Play. This is a measure of the number of batted balls that safely fall in for a hit (not including home runs). The exact formula we use is (H-HR)/(AB-K-HR+SF) This is similar to DER, but from the batter's perspective.

BB (Link)

Bases on Balls, otherwise known as walks.

BB/G (Link)

Walks Allowed per games pitched. This stat is based on the number of walks allowed divided by total number of batters faced, times the average number of batters per game in that specific league (generally around 38 batters a game). This is preferable to BB/IP, since better pitchers will face less batters per inning.

BFP (Link)

Batters Faced by Pitcher. The pitching equivalent of Plate Appearances for batters.

BIZ (Link)

Balls in Zone, a fielding stat that represents the total number of balls that were batted into a fielder's zone while he was in the field.

BR, or Base Runs(Link)

A run contribution formula created by David Smyth, which quantifies the number of runs contributed by a batter. The fundamental formula for Base Runs is (baserunners times scoring rate) plus home runs. This article, among others, has found that Base Runs is the best run contribution formula. You can read more about it in this Wiki entry.

Clutch (Link)

One of the benefits of the Win Probability Added approach is that it provides a useful way to measure the criticality of a situation, via the Leverage Index statistic. For example, in one of the breakouts at Baseball Reference, you can view a player's performance split into situations of low, medium and high Leverage Indices to see how much better or worse he performed in critical situations.

"Clutch," as provided at both Fangraphs and Baseball Reference, is a single number that sums up his Leverage Index splits. The specific definition starts with a player's overall WPA divided by his overall Leverage Index. In general, a higher Leverage Index will accentuate his WPA contribution (good or bad), and dividing by LI "normalizes" it.

The stat then looks at each individual play and divides the player's WPA in that specific play by the LI of that specific play. It then sums up all the plays in the year. Finally, it subtracts this figure (the sum of each specific WPA/specific LI) from the first figure (overall WPA/overall LI). The result is an index of whether the player did better or worse in "clutch" situations. Zero is neutral, a positive number is "clutch" and a negative number isn't.

CS (Link)

Caught Stealing

CWS (Link)

Often stands for College World Series, but it's Career Win Shares in our stats. Our methodology for calculating Win Shares after 2003 differs somewhat from Bill James's original formula.

DER (Link)

Defense Efficiency Ratio. The percent of times a batted ball is turned into an out by the teams’ fielders, not including home runs. The exact formula we use is (BFP-H-K-BB-HBP-Errors)/(BFP-HR-K-BB-HBP). This is similar to BABIP, but from the defensive team's perspective. Please note that errors include only errors on batted balls.

DP (Link)

Double Plays

DPS (Link)

Double Plays Started, in which the fielder typically gets only an assist.

DPT (Link)

Double Plays Turned, in which the fielder records both an assist and an out.

ERA (Link)

Earned Run Average. Number of earned runs allowed divided by innings pitched multiplied by nine.

ERA+ (Link)

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. The specific formula divides the league ERA by the pitcher's ERA (and adjusts for ballpark). So an ERA+ of 125, for instance, means that the league ERA was 25% higher than the pitcher's ERA (which means that the pitcher's ERA was 80% of the league ERA). Careful with those ratios.

Expected Win Shares(Link)

Expected Win Shares are a playing time metric. They are the number of Win Shares a player would be expected to accumulate, given his playing time (including plate appearances, innings in the field, and innings pitched), if he were an average player. Expected Win Shares is a baseline for evaluating Win Shares totals, as in Win Shares Above Bench.

FE (Link)

Fielding Errors, as opposed to Throwing Errors (TE)

FIP (Link)

Fielding Independent Pitching, a measure of all those things for which a pitcher is specifically responsible. The formula is (HR*13+(BB+HBP-IBB)*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. The theory behind FIP was first articulated by Voros McCracken and the exact formula was invented by Tangotiger.

FPct (Link)

Fielding Percentage, or the number of fielding chances handled without an error. The formula is (A+PO)/(A+PO+E).

G (Link)

Games played.

GB% (Link)

The percent of all batted balls (not just outs) that are ground balls.

G/F (Link)

G/F stands for Ground ball to Fly ball Ratio. It is the number of ground balls divided by the number of fly balls (but not line drives) hit by the batter or allowed by the pitcher. It includes all batted balls, not just outs.

GIDP (or GDP) (Link)

The number of times a batter Grounded Into Double Plays.

GPA (Link)

Gross Production Average, a variation of OPS, but more accurate and easier to interpret. The exact formula is (OBP*1.8+SLG)/4, adjusted for ballpark factor. The scale of GPA is similar to BA: .200 is lousy, .265 is around average and .300 is a star. A simple formula for converting GPA to runs is PA*1.356*(GPA^1.77).

GS (Link)

Games Started, a pitching stat.

HRA (Link)

Home Runs Allowed, also a pitching stat.

HR/Fly or HR/F (Link)

Home Runs as a percentage of outfield fly balls. The home run totals are adjusted by the home ballpark's historic home run rates. Research has shown that about 11% to 12% of outfield flies are hit for home runs. For pitchers, significant variations from 11% are probably the result of "luck," but for hitters this stat is more indicative of a true skill (hitting the ball hard!).

HR/G (Link)

Home Runs Allowed per games pitched. This stat is based on the number of home runs allowed divided by total number of batters faced, times the average number of batters per game in that specific league (generally around 38 batters a game).

IBB (Link)

Intentional Base on Balls.

IF/Fly or IF/F (Link)

The percent of flyballs that are infield flies. For some pitchers, inducing infield flies may be a repeatable skill.

ISO (Link)

Isolated Power, which measures the “true power” of a batter. The formula is SLG-BA.

K (Link)

Strikeouts

K/G (Link)

Strikeouts per games pitched. This stat is based on the number of strikeouts divided by total number of batters faced, times the average number of batters per game in that specific league (generally around 38 batters a game).

L (Link)

Losses

LD% (Link)

Line Drive Percentage. Baseball Info Solutions tracks the trajectory of each batted ball and categorizes it as a groundball, fly ball or line drive. LD% is the percent of batted balls that are line drives. Line drives are not necessarily the hardest hit balls, but they do fall for a hit around 75% of the time.

Leverage Index (Link)

Leverage Index, invented by Tangotiger, is a measure of how critical a specific batting situation is. One (1) is average, anything above one is more critical and anything less than 1 is less critical. You can read more about Leverage Index in Tango's three-part series on THT: (Part One, Part Two, and Part Three).

LOB and LOB% (Link)

LOB stands for Left On Base. It is the number of runners that are left on base at the end of an inning. LOB% is slightly different; it is the percentage of baserunners allowed that didn't score a run. LOB% is used to track pitcher's luck or effectiveness (depending on your point of view). The exact formula is (H+BB+HBP-R)/(H+BB+HBP-(1.4*HR)). You can read more about LOB% in this article.

Marcels (Link)

The Marcels are a simple way of calculating a player forecast. Named after the monkey from Friends (so simple a monkey could do them), they simply consist of averaging a player's previous experience (with greatest weight on the most recent years) and regressing to the major league average depending on the number of years the player has been in the majors. This is done for each component (home runs, doubles, walks, etc.) A simple aging factor is applied, but no park factor. The Marcels, made available by Tangotiger, are explained in this post.

OBP (Link)

On Base Percentage, the proportion of plate appearances in which a batter reached base successfully, including hits, walks and hit by pitches. The specific formula is (H + BB + HBP) divided by (AB + BB + HBP + SF). OBP is also a powerful performance metric when interpreted as the percentage of times the batter didn't make an out.

OOZ (Link)

Out Of Zone, or the total number of outs made by a fielder on balls hit outside of his zone.

Op (Link)

Save Opportunities

OPS (Link)

On Base plus Slugging Percentage, a crude but quick measure of a batter’s true contribution to his team’s offense. See GPA for a better approach.

OPS+ (Link)

OPS measured against the league average, and adjusted for ballpark factors. An OPS+ over 100 is better than average, less than 100 is below average.

P (Link)

Invented by Doug Drinen in the late 1990's, P measures the criticality of a reliever's appearance. It is computed by estimating the impact a successful relief appearance would have on a team's WPA. You can read more about P in The One About Win Probability and Closer.

P/GS (Link)

Pitches per Game Started (only shown for pitchers who have started in 100% of their appearances).

P/PA (Link)

Pitches per Plate Appearance.

PA (Link)

Plate Appearances, or AB+BB+HBP+SF+SH.

PO (Link)

Putouts, the number of times a fielder recorded an out in the field. First basemen and outfielders get lots of these. From a pitching perspective, PO stands for pick offs -- the number of times a pitcher picks a baserunner off a base.

POS (Link)

Position played in the field.

PR (Pitching Runs) (Link)

Invented by John Thorn and Pete Palmer, this is a measure of the number of runs a pitcher saved compared to average. The formula is league-average RA/IP minus park-adjusted RA/IP, times total innings pitched. This is the same formula as Lee Sinins' RSAA (see below).

PRC (Pitching Runs Created) (Link)

A new stat, created by our own David Gassko. The notion behind Pitching Runs Created is that a run saved is worth more than a run scored, and PRC puts runs saved on the same scale as runs scored. You can directly compare PRC to a batter's Runs Created to gauge each player's relative value to his team. You can read more about Pitching Runs Created in the introductory article and the follow-up article.

PrOPS(Link)

PrOPS stands for "Predicted OPS." It was developed by J.C. Bradbury amd introduced in this article. PrOPS isn't really a new stat; it's a formula for predicting what a player's OPS is likely to be in the future based on his batted balls, strikeouts, home runs and walks.

Pythagorean Record (Link)

A formula for converting a team’s Run Differential into a projected Won/Loss record. The formula is RS^2/(RS^2+RA^2). Teams’ actual won/loss records tend to mirror their Pythagorean records, and variances can usually be attributed to luck.

You can improve the accuracy of the Pythagorean formula by using a different exponent (the 2 in the formula). In particular, a sabermetrician named US Patriot discovered that the best exponent can be calculated this way: (RS/G+RA/G)^.287, where RS/G is Runs Scored per game and RA/G is Runs Allowed per game. This is called the PythagoPat formula.

R (Link)

Runs Scored and/or Allowed.

R/G (Link)

Runs Scored Per Game. Literally, R divided by games played.

RA (Link)

Runs Allowed Per Nine Innings. Just like ERA, but with unearned runs, too.

RBI (Link)

Runs Batted In

RC (Link)

Runs Created. Invented by Bill James, RC is a very good measure of the number of runs a batter truly contributed to his team’s offense. The basic formula for RC is OBP*TB, but it has evolved into over fourteen different versions. We use the most complicated version, which includes the impact of hitting well with runners in scoring position, and is adjusted for ballpark impact. RC/G refers to Runs Created Per Game, which Runs Created divided by the number of outs made by the batter, times 27.

RCAA (Link)

Runs Created Above Average. A stat invented and tracked by Lee Sinins, the author of the Sabermetric Baseball Encyclopedia. Lee calculates each player’s Runs Created, and then compares it to the league average, given that player’s number of plate appearances. Lee uses a different version of RC than we do, though the two are very similar.

RF (Link)

Range Factor, a measure of the total chances fielded in a player’s playing time. The formula we use is 9*(PO+A)/Innings in Field.

Replacement Level (Link)

When you talk about stats, how do you compare players? You can compare batters and pitchers to the equivalent of "zero" runs (though being above zero obviously has different implications for batters and pitchers). You can compare players to average too. In fact, you have to compare fielders to average if you use advanced fielding stats such as Ultimate Zone Rating. So when you combine batting, pitching and fielding stats into one "uberstat" such as Win Shares or Wins Above Replacement, you have to compare players to average.

And that creates a problem. Comparisons to average means that a player who plays just one game and is average in that game is just as valuable as the player who played an entire average season. Doesn't make sense. So we compare players to replacement level instead. Replacement level is usually thought of as the level at which a player can be easily replaced by a bench player, a freely available free agent or someone else. Replacement levels are also useful for salary analysis in which it is assumed that a player who performs at a replacement level should receive a minimum salary.

There are a lot of ways to calculate replacement levels. Here is one, presented by our own Sean Smith. We use a different approach for Win Shares Above Bench. There are several legitimate approaches. The key thing to remember is that replacement levels will differ by position, because it is harder to find a catcher than a first baseman (for example).

RS (Link)

Runs Scored

RSAA (Link)

Runs Saved Above Average. This stat, which is also tracked and reported by Lee Sinins, is a measure of a pitcher’s effectiveness and contribution. The formula is RA/IP minus league-average RA/IP, times total innings pitched.

Run Differential (Link)

Runs Scored minus Runs Allowed.

RZR (or ZR)(Link)

Revised Zone Rating is the proportion of balls hit into a fielder's zone that he successfully converted into an out. Zone Rating was invented by John Dewan when he was CEO of Stats Inc. John is now the owner of Baseball Info Solutions, where he has revised the original Zone Rating calculation so that it now lists balls handled out of the zone (OOZ) separately (and doesn't include them in the ZR calculation) and doesn't give players extra credit for double plays (Stats had already made that change). We believe both changes improve Zone Ratings substantially. To get a full picture of a player's range, you should evaluate both his Revised Zone Rating and his plays made out of zone (OOZ). You can read more about the Revised Zone Ratings in this article.

SB (Link)

Stolen Bases

SB% (Link)

The percent of time a runner stole a base successfully. The formula is SB/SBA.

SBA (Link)

Stolen Bases Attempted.

SBA/G (Link)

Stolen Base Attempts per 9 innings played.

ShO (Link)

Shutouts

SLG and SLGA (Link)

Slugging Percentage. Total Bases divided by At Bats. SLGA stands for Slugging Percentage Against. It represents SLG from the pitcher's perspective.

SO (Link)

Strikeouts

Sv (Link)

Saves

Sv% (Link)

Saves divided by Save Opportunities

TB (Link)

Total Bases, calculated as 1B+2B*2+3B*3+HR*4.

TBA (Link)

Total Bases Allowed. A pitching stat.

TE (Link)

Throwing Errors, as opposed to Fielding Errors (FE).

TotalZone (Link)

Sean Smith's fielding evaluation system. TotalZone is similar to UZR in that it evaluates fielders on a plus/minus scale compared to average. The difference is that TotalZone adapts to the data available, so that it can be applied to any period in baseball history. You can read more about TotalZone here, though Sean continues to refine his system. TotalZone is available at Baseball Reference and is part of their WAR calculations.

UER (Link)

Unearned Runs

UERA (Link)

Unearned Run Average, or the number of unearned runs allowed for each nine innings pitched.

UZR (Link)

Ultimate Zone Rating. The name was coined by John Dewan in the late 1990's and the stat was created by Mitchel Lichtman in the early 2000's. Essentially, UZR looks at the trajectory and speed of every batted ball and, based on overall major league averages, assigns a probability that a certain position will field it. If a player at that position fields it, he gets credit above the overall major league average. If he doesn't, he gets negative credit.

UZR is adjusted in many ways and includes other fielding metrics. There are also other fielding stats, such as John Dewan's Plus/Minus and Sean Smith's TotalZone, that essentially do the same thing (but with different approaches to the data). It gets complicated very quickly. But the bottom line is that a player with zero UZR is an average fielder at his position. A positive number indicates he was an above-average fielder, and a negative number indicates that he was below average.

W (Link)

Wins

WAR (Link)

Wins Above Replacement. This is a metric that combines a player's contributions on offense and defense and then compares him to the appropriate replacement level for his position. Currently, there are two versions in general use, at Baseball Reference and Fangraphs. The framework was developed by Tangotiger.

WHIP (Link)

Walks and Hits Per Inning Pitched, a variant of OBP for pitchers. This is a popular stat in rotisserie baseball circles.

wOBA (Link)

Weighted On Base Average. wOBA was developed by Tangotiger and introduced in The Book: Playing the Percentages in Baseball (essential reading for all sabermetric baseball fans). wOBA is a rate stat, calibrated to follow the same scale as On-Base Percentage, that captures the value of all of a hitter's contributions.

wOBA uses a "linear weights" approach in that it assigns a coefficient to every contribution made by a batter (.9 for a walk, 1.95 for a home run, etc.) and then divides by his plate appearances. You can also alter the coefficients based on the batter's run environment.

WPA (Link)

Win Probability Added. 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. You can read more about WPA in The One About Win Probability.

WP+PB/G (Link)

Wild Pitches and Passed Balls per Nine Innings played. A fielding stat for catchers.

wRC (Link)

Weighted Runs Created. An estimate of the number of runs created by a player for his team, based on the same math behind wOBA. While wOBA is a rate stat, wRC is a counting stat, affected by playing time.

wRC+ (Link)

Weighted Runs Created Plus. This measures how a player's wRC compares to the league average, adjusted for ballpark. 100 is average, 120 is twenty percent above average and 80 is 20 percent below average.

WS (Link)

Win Shares. Invented by Bill James. Win Shares is a very complicated statistic that takes all the contributions a player makes toward his team’s wins and distills them into a single number that represents the number of wins contributed to the team, times three. We have tweaked James’ original formula somewhat.

WSAB (Link)

Win Shares Above Bench, or Baseline. WSAB is a refined approach to Win Shares, in which each player's total Win Shares are compared to the Win Shares an average bench player would have received, given that player's time at bat, on the mound or in the field.

This is an important adjustment to Win Shares, as we discovered during the 2003-2004 offseason. The bench player approach is explained in this article. It is essentially 75% of Expected Win Shares for all players except Starting Pitchers, for whom it is 60% of Expected Win Shares.

WSP (Link)

Win Shares Percent, a Win Shares "rate stat" -- a measure of the player's contribution, given his playing time. The math is WS/(2*ExpWS). Expected Win Shares are the number of Win Shares an average player contributed, given that particular player's time at bat, on the mound or in the field.

xFIP (Link)

Expected Fielding Independent Pitching. This is an experimental stat that adjusts FIP and "normalizes" the home run component. Research has shown that home runs allowed are pretty much a function of flyballs allowed and home park, so xFIP is based on the average number of home runs allowed per outfield fly. Theoretically, this should be a better predicter of a pitcher's future ERA.

Zone (Link)

The areas on a ballfield in which at least 50% of batted balls are handled for outs. Zones are standardized and defined separately for each position.

Baseball Statistics also has a very good glossary, including many stats that aren't listed here. Also, be sure to check out Fangraphs' Sabermetric Library.