While the list may seem exhaustive, there are plenty of statistics not listed here, but I hope to have covered every statistic you might see on our blog.
/9 (Per Nine Innings)
This is a common notation used in conjunction with another statistic to measure that statistic as a rate per nine innings, in order to compare players with different amount of playing time more accurately. It is used almost exclusively with pitching statistics
A single is a ball hit by the batter which allows him to reach first base safely that does not result in another player being forced out at a different base. If, however, in the judgment of the official scorer, he only makes it to first base as the result of an error by a fielder, he does not get credit for a single.
A double is counted when a batter reaches second base safely as a result of a ball he hit. If, however, in the judgment of the official scorer, he only makes it to second base as the result of an error by a fielder or an attempt, successful or not, to get a different runner out, he does not get credit for a double.
A triple is counted when a batter reaches third base safely as a result of a ball he hit. If, however, in the judgment of the official scorer, he only makes it to second base because of an error by a fielder or an attempt, successful or not, to get a different runner out, he does not get credit for a triple.
This functions the same way as Opp and can be used interchangeably. Opp is probably more common though. It can be added to batting average (BAA), slugging (SLGA), on base percent (OBA), or any other statistic.
An assist is recorded when a fielder throws to another fielder who either tags or forces a runner out. If the play requires multiple throws, such as a relay from the outfield, only one thrower gets credited with an assist.
AB (At Bat)
An at bat is a smaller subset of plate appearances. Plate appearances that do not count as at bats are the following: walks, hit-by-pitches, sacrifice flies, sacrifice hits, and the rarely seen defensive interference.
AVG (Average, BA, Batting Average)
Formula: ( H / AB)
Batting average is a measure of how often a player gets a hit when he puts the ball into play. It is a very traditional statistic that has been used as long as baseball has been around. It is not as good a measure of a player's skill, however, as many newer statistics because it does not factor in a player's power or his general ability to get on base by drawing walks.
BABIP (Batting Average on Balls in Play)
Formula: ( H / (AB - HR - K))
BABIP measures how often players get a hit on balls that they hit into the field of play. For both batters and pitchers, it is used to determine whether a player's batting average (or batting average against) is skill or luck-based, as a very high BABIP indicates unsustainable good luck and a very low BABIP indicates unsustainable bad luck. The reasoning behind this is that, with a given batted ball profile, players have little control over their BABIP.
Batted Ball Profile
A batted ball profile is the distribution of fly balls (FB%), ground balls (GB%), and line drives (LD%) a player hits.
BB (Walk; Base on Balls)
A walk is earned by a batter when the pitcher throws four balls in one plate appearance before the batter puts the ball into play and before the pitcher earns a strikeout. Walks do not count as at bats, but do count as plate appearances.
BB% (Walk Percent)
Formula: ( BB / PA * 100 )
Walk percent is quite simply the percentage of plate appearances that result in a walk.
BIP (Ball in Play)
A Ball in Play is any ball hit into the field by a batter. This statistic is rarely, if ever, seen on its own, but is used to determine other statistics.
A balk is an illegal move made by the pitcher in an attempt to deceive a baserunner(s). A balk can be called for many reasons, including but not limited to: not coming to a set position before pitching, faking a pitch to home and throwing to base (and vice versa), and not stepping off the pitching rubber before throwing to a base. When called, it allows all runners to move up one base, including letting a runner at third base automatically score, and gives the batter one additional ball. It is a convoluted rule that is not uniformly applied and often causes controversy when it is called.
BS (BSV, BLSV, Blown Save)
A pitcher is given a blown save when he comes into a save situation and allows the game to become tied or the opposing team to go ahead.. A save situation occurs when the pitcher's team is ahead by three runs or fewer past the sixth inning, as long as the pitcher enters the game at the start of the ninth inning or earlier. If he enters partway through the ninth inning, the tying run must be on-deck for it to count as a save situation. Unlike with earned runs, a pitcher is given a blown save based on whether he was pitching when the tying run scored, not based on who allowed that runner to get on base.
CG (Complete Game)
A pitcher is credited with a complete game when he pitches the entire game. This usually means he pitched nine innings, but in a road loss he can pitch eight innings and be awarded a complete game. On the other hand, if a game goes into extra innings, the pitcher does not get credit for a complete game unless he pitches the entire game, not just nine innings.
Contact% (Contact Percent)
Contact Percent is a measure of how often a player makes contact when he swings at a pitch. Foul balls and balls in play both count as making contact; only when he swings and misses is a batter defined to have not made contact.
CS (Caught Stealing)
A caught stealing is a stolen base attempt in which the baserunner is thrown out, usually by the catcher. However, a caught stealing is awarded any time the runner gets out when running toward the succeeding base on a stolen base attempt.
CS% (Caught Stealing Percent)
Formula: ( CS / (SB + CS))
This is the percent of times a catcher throws out opposing baserunners on stolen base attempts.
Defensive indifference is scored when the defense allows a runner to steal a base without making an attempt to get him out. Sometimes a player can steal a base without a throw, and it is not necessarily called defensive indifference. It is up to the official scorer to determine if the team intended to prevent him from stealing. As a result, defensive indifference only happens when the defensive team is up by many runs.
DP (Double Play)
A double play occurs when the fielding team gets two outs on one play. This can only occur when at least one runner is already on base. It can be the result of two force outs at two different bases, a catch and a force out, a catch and a tag on a runner attempting to advance, or any other fashion in which two outs are recorded on the same play.
A fielder is given an error when, in the judgment of the official scorer, a batter reaches base or a baserunner is able to move up on extra base as the result of a poor fielding play by that fielder. If a fielder makes a bad play but no runners move up and the batter doesn't reach base as a result, no error can be charged. As a rule, mental mistakes and miscommunications are never recorded as errors. For example, if two players look at a ball that drops between them and neither catches it because they thought the other fielder would, no error is charged.
ER (Earned Run)
An earned run is charged to a pitcher when, in the judgment of the official scorer, the run would have scored regardless of any errors or passed balls that occurred. If there are no errors or passed balls in an inning, all runs that score that inning must be earned. On the other hand, any batter who only reaches base because of an error and any runs that score after the third out would have been recorded, if not for an error, cannot count as earned runs. Runs that are not earned are referred to as unearned runs.
ERA (Earned Run Average)
Formula: ( (ER / IP) * 9)
Earned Run Average is a statistic devised in order to show how well a player pitched. It is simply a formula that calculates earned runs allowed as a rate per nine innings. It is the most commonly-used statistic to evaluate a pitcher's performance, but it has flaws that many people overlook. For instance, two pitchers can allow the same number and type of hits and walks in an inning, but they can allow different amounts of runs as the result of baserunners getting out on the basepaths, taking extra bases, the order in which those hits happen, or fielders making particularly good or bad plays that are not scored as errors.
This statistic is ERA, but on a different scale and with context taken into account. To illustrate, an ERA of 3.00 in a league where the average ERA is 4.00 is pretty good, while an ERA of 3.00 in a league where the average ERA is also 3.00 is merely average. That level of fluctuation doesn't usually happen, but it illustrates the usefulness of adjusting ERA based on the run-scoring environment. The scale sets 100 as average, and each number above or below that is 1 percentage point better or worse than average, respectively.
FB% (Fly Ball Percent)
Formula: ( FB / BIP)
This is a measure of the percent of time a ball in play is a fly ball. The designation between ground balls, fly balls, and line drives is not always unanimous. In general, the BABIP for fly balls is slightly lower than for ground balls, and much lower than that for line drives.
FIP (Fielding Independent Pitching)
Formula: ((( (13*HR) + (3*BB) - (2*K)) / IP) + 3.10)
Note: The last number is a constant that is usually around 3.10 but changes based on the league and year.
The idea behind Fielding Independent Pitching is to correct for the mistakes inherent in ERA, most importantly eliminating any reliance on the ability of the fielders to evaluate the pitcher's performance. It is set on the same scale as ERA, but uses only statistics that are not affected by defense in its calculation, specifically strikeouts, walks, and home runs. Through a relatively complicated formula, it determines what a pitcher's ERA should be based on the aspects he controls.
A force out occurs when a fielder tags a base that a baserunner or the batter must run to, causing him to be called out.
FP (Fielding Percent)
Formula: ( A + PO / TC )
Fielding Percent is a measure of how good a player is at fielding. It divides the numbers of outs he makes by the total chances he has in the field, which includes the number of outs he makes and the number of errors he commits. It is a commonly used fielding metric, but not a very good one. That is because it does not penalize players who do not have the range to make plays that another fielder can, and instead looks at only one statistic (errors) that is very subjectively determined by the official scorer.
F-Strike% (First-Pitch Strike Percent)
First-Pitch Strike Percent is, as the name suggests, the percentage of times a pitcher throws the first pitch of a plate appearance for a strike. Swinging strikes, called strikes, foul balls, and balls hit into play all count as strikes.
G (GP, Game, Game Played)
A player gets credit for a game played any time he appears in a game, be it as a starter, a defensive replacement, a pinch hitter, or a pitcher.
Formula: ( 50 + ( (3*IP) + (2*ICA4) + (1*K) + (2*H) - (4*ER) - (2*UER) - (1*BB) ))
ICA4: Innings completed after the fourth inning
UER: Unearned runs
Game Score is a pitching statistic developed by Bill James to evaluate individuals starts made by starting pitchers. Through a complex formula, it gives a one-number summary of how well a pitcher pitched in a specific game. A game score above 50 is considered to be a quality start, though the specific quality start statistic in this glossary has nothing to do with game score.
GB% (Ground Ball Percent)
Formula: ( GB / BIP)
This is a measure of the percent of time a ball in play is a ground ball. The designation between ground balls, fly balls, and line drives is not always unanimous. In general, the BABIP for ground balls is higher than for fly balls, but much lower than that for line drives.
GDP (GIDP, Ground into Double Play)
A batter grounds into a double play when he hits a ball which allows the defense to record two outs on one play. The batter usually gets out, but he can reach first base on a double play if two other runners get out. He is still considered to have grounded into a double play in that situation.
GF (Games Finished)
A game finished is credited to a pitcher when he is the last pitcher to pitch in a game for his team. In every game, one pitcher from each team gets credit for a game finished.
GS (Game Started)
A game started is credited to a pitcher who is the first pitcher to pitch in a game for his team. In every game, one pitcher from each team gets credit for a game finished. This is also used for position players, though less often. In order to get credit for a game started, a player must hit the first time his spot in the batting order comes up or play a full inning on defense, whichever comes first.
A hit is a ball batted by the hitter which allows him to reach at least first base safely that does not result in another player being forced out. If, however, in the judgment of the official scorer, the batter only reached first base safely as the result of an error by a fielder or an attempt, successful or not, to get a different runner out, the batter is not awarded a hit. Each hit must count as one, and only of the following: a single, double, triple, or home run.
HB (Hit Batsman, Hit Batter)
A hit batter is a pitching statistic which is recorded any time a batter reaches first base as the result of being hit by a pitch.
HBP (Hit By Pitch)
A hit-by-pitch is a batting statistic which is recorded any time a batter reaches first base as the result of being hit by a pitch. A hit-by-pitch does not count as an at bat. If a batter swings and gets hit by a pitch, the pitch is counted as a strike and the batter does not get to automatically go to first base.
HLD (H, Hold)
Holds are not an official statistic, and thus there are different ways of defining a hold. In general it is awarded to a pitcher who enters the game with his team leading and maintains the lead. This can result in situations where a pitcher gets no one out and is still credited with a hold, though some definitions of a hold require the pitcher to get at least one out.
HR (Home Run)
A home run is a fair ball that clears the field of play on a fly, allowing all players on base and the batter to score one run each. If the ball hits anything other than a defensive player before clearing the field of play, such as the ground or the wall, it is an automatic double.
HR/FB (Home Runs Per Fly Ball)
Formula: ( HR / Fly Balls)
As the name suggests, this statistic measures what percentage of fly balls a batter hits are home runs. All home runs are counted as fly balls, and thus are included in the denominator. The idea is that players have a relatively consistent true talent HR/FB, so a ratio much higher or lower than their career percentage suggests they will not continue to hit home runs at that rate.
IBB (Intentional Walk, Intentional Base on Balls)
An intentional walk, which also counts as a walk, is a walk that the pitcher issues intentionally. Pitchers will do this in order to pitch against a worse hitter or to set up a force out at a base(s).
IFFB% (Infield Fly Ball Percent)
Formula: (Infield Fly Balls / Fly Balls)
IFFB% is a measurement of how often a player hits infield fly balls, or fly balls that are caught in the infield. A higher or lower-than-usual IFFB% can be used to explain a HR/FB% that has dropped or risen, respectively, and can be used as evidence that the HR/FB% will not regress to the player's career rate.
IP (Inn, Innings Pitched)
Each out recorded by a pitcher counts as 1/3 of an inning pitched. Each full inning, therefore, counts as 1 inning pitched. All outs that are recorded while a pitcher is in the game count as 1/3 of an inning pitched, even if the out is recorded on a baserunner who batted before that pitcher enters the game. For example, if a pitcher enters the game with a runner on first base, and then picks off the batter even before throwing a pitch, the pitcher is credited with 1/3 IP without actually throwing a pitch. Innings (abbreviated Inn) is also used to measure the total time a fielder spends at a position.
ISO (IsoP, Isolated Power)
Formula: ( SLG - AVG )
Isolated Power is used to measure a player's raw power. While Slugging Percent inherently takes average--and thus singles, which do not really demonstrate power--into account, ISO counts only extra base hits. While Slugging Percent may better judge how well a player has played, ISO is a better statistic for determining the amount of power a batter his displayed. Slugging Percent is a much more mainstream statistic, though.
IsoD (Isolated Discipline)
Formula: ( OBP - AVG)
Isolated Discipline attempts to isolate a player's ability to get on base via walks, and thus to isolate his plate discipline. Although the idea behind Isolated Discipline is the same as that behind Isolated Power, it is not used nearly as much; BB% is the main statistic to measure a player's plate discipline.
K (SO, Strikeout)
A strikeout is recorded when a batter gets three strikes in a plate appearance before either putting the ball into play or getting four balls called. A foul ball with two strikes is counted as a strike, but not as the third strike and the batter continues batting with two strikes. A foul bunt in that situation does result in a strikeout. A player may reach base on either a swinging third strike or a called third strike if the catcher does not catch the pitch before it hits the ground and the batter is able to run to first base before being tagged or forced out.
K% (SO%, Strikeout Percent)
Formula: ( K / AB )
Strikeout Percent is simply the percentage of at bats in which a player strikes out for a hitter, or the percentage of opponents' at bats that end in a strikeout for a pitcher. Since the denominator for K% is at bats and the denominator for BB% is plate appearances, a player can actually have more walks than strikeouts but have a higher K% than BB%.
K/BB (SO/BB, Strikeout-to-Walk Ratio)
Formula: ( K / BB )
Strikeout-to-Walk Ratio is used most commonly for pitchers, but is also used sometimes for batters. It is a measure of how many strikeouts a player gets for every walk that he issues (or earns, for a hitter). K/BB ratio is especially useful because it combines two important aspects of pitching without having to rely on any variation in fielding ability.
A pitcher is credited with a loss if the the go-ahead run is a player he allowed to reach base. That is to say, if a pitcher allows a batter to reach base, even via an error, and that player then scores the run that first breaks the tie between the two teams, and the pitcher's team never ties the game or takes the lead after that, the pitcher earns a loss.
LD% (Line Drive Percent)
Formula: (LD / BIP)
This is a measure of the perecent of time a ball in play is a line drive. The designation between ground balls, fly balls, and line drives is not always unanimous. In general, the BABIP for line drives is significantly higher than for ground balls and fly balls.
League is used before another statistic to indicate that it is the league average for that year for that statistic.
LOB% (Left on Base Percent)
LOB% is the percentage of runners a pitcher allows to reach base that do not wind up scoring. Home runs are left out of this statistic altogether. Under the theory that a pitcher does not control when he gives up hits, but merely on average how many he will give up, a high LOB% indicates the pitcher has been the recipient of good luck, and a low LOB% indicates the opposite.
OBP (On Base, On Base Percent, On Base Percentage)
Formula: ((H + BB + HBP) / PA)
OBP is a statistic designed to measure a player's ability to get on base. It divides the three ways a player can 'earn' his way on base--hits, walks, and hit-by-piches--by the number of times a player bats. When a player reaches base via an error or on a force out, he does not get credit for reaching base in terms of his On Base Percent.
OPP (Opponent, Opposing)
OPP (or, more commonly, Opp) is often used before a pitching statistic to denote an opponent's statistic. For instance, Opp BA denotes the batting average a pitcher has allowed opposing batters to hit for.
OPS (On Base Plus Slugging Percent)
Formula: (OBP + SLG)
OPS is simply On Base Percent and Slugging Percent added together. The idea is to give a one-statistic summary of how good a player is. It combines his ability to get on base (On Base Percent), his ability to hit for power (Slugging Percent), and his ability to hit for average, which inherently affects both component statistics. It is relatively simple and commonly used.
Like ERA+, OPS+ simply converts OPS to a different scale and evaluates OPS based on the run-scoring environment. This makes it easier to compare players from different eras, when run-scoring averages may have varied widely. An OPS+ of 100 is average, while each point above or below that is one percentage point better or worse than average, respectively.
O-Contact% (Outside Strike Zone Contact Percent)
O-Contact % is the percentage of time a hitter makes contact with a pitch out of the strike zone, given that he swings at the pitch.
O-Swing% (Outside Strike Zone Swing Percent)
O-Swing % is the percentage of time a hitter swings at a pitch that is out of the strike zone.
PA (Plate Appearance)
A plate appearance is recorded any time a player bats. Every at bat is a plate appearance, but not all plate appearances count as at bats.
PB (Passed Ball)
A passed ball occurs when a catcher allows a pitch to get away from him in such a manner that at least one baserunner moves up one or more bases. If no baserunners move up, a passed ball cannot occur. The distinction between a passed ball and a wild pitch is that a passed ball is deemed--by the official scorer--to be a pitch the catcher should have blocked and thus prevented the runner(s) from advancing.
A pickoff is when a pitcher throws to a base occupied by a runner, rather than throwing a pitch, and that runner is tagged out. If the runner only dives or steps toward the base he was already on, he is not considered to be caught stealing. If he breaks toward the next base, he is considered to be caught stealing. Either way it is a pickoff.
PO (Put Out)
A put out is recorded when a player is directly responsible for getting a baserunner out. This is accomplished either by tagging a base the runner is forced to (a force out), by tagging a baserunner, or by catching a batted ball in the air. One and only one player must get a put out for every out that is recorded. This means, on a strikeouts, a catcher records the put out as long as he catches the pitch; if he doesn't he must either tag the batter or throw to first to complete the strikeout.
QS (Quality Start)
A quality start is intended to indicate that a pitcher had a good start. The definition of a quality start is that a pitcher starts the game and pitches at least 6 innings and allows 3 or fewer earned runs. It is not necessarily a very good statistic, since the cutoff is fairly arbitrary, as a pitcher who throws 5 2/3 innings and allows 0 runs does not get a quality start, while a pitcher who throws 6 innings and allows 5 runs, but just 3 earned runs, does get a quality start.
QS% (Quality Start Percent)
Formula: ( QS / GS )
Quality Start Percent is the percentages of starts a pitcher makes that result in quality starts.
A run is scored any time a player crosses home plate before the third out of the inning is recorded, as long as there is not a force out completed to end the inning on that play. Each run that scores is credited to the individual player who scored that run.
RA (Runs Allowed)
A pitcher is considered to have allowed a run anytime a player who originally reached base while he was pitching scores, regardless of whether or not he is still pitching. Likewise, if a pitcher allows a runner to score who was already on base when he entered the game, that run is not charged to that pitcher. Each run that scores must be charged to an individual pitcher.
RBI (Run Batted In)
A hitter earns an RBI when the result of his plate appearance allows a baserunner(s) to score, whether with a walk, a hit, a hit-by-pitch, a ground out, a fly out, etc. No batter can get an RBI on a play in which he grounds into a double play and a batter does not get an RBI when, in the judgment of the official scorer, the runner would not have scored if not for an error. Batters do get an RBI for scoring themselves on a home run.
RF (Range Factor)
Formula: ( (A + PO) / Inn) ) or ( (A + PO) / G) )
Range Factor is a fielding statistic created by Bill James that places value on plays made rather than on errors made. The formula is simply the number of outs recorded per innings or per game, depending on which site you are looking at. It is also sometimes used as a rate stat per nine innings (RF/9).
ROE (Reached on Error)
A reached on error is awarded to a batter who reaches base because of an error when he would not have otherwise, in the judgment of the official scorer. A reached on error counts as an at bat and a plate appearance, but does not count as a hit or as reaching base.
RS (Runs Scored)
Runs scored is the abbreviation used for team runs scored over the course of a season. RS is never used for individual players.
SB (Stolen Base, Steal)
A stolen base occurs when a baserunner moves from one base to another without the batter hitting the ball. It almost always occurs during a pitch, but can happen any time the pitcher is on the mound and holding the ball if the defensive players are not paying attention. Sometimes a player will move up a base on a pitch and not get credit for a steal. This happens if he was only able to advance because of a wild pitch or passed ball, or because of defensive indifference.
SB% (Stolen Base Percent)
Formula: ( SB / (SB + CS))
This is the percent of times a player is successful on a stolen base attempt. A player who is picked off and does not break toward the next base is not considered to have been caught stealing, and thus is not counted as having attempted to steal a base.
SF (Sacrifice Fly)
A hitter is credited with a sacrifice fly when at least one runner scores on his fly out. Almost always only one player, who is on third base, can score on sacrifice fly, but in unusual circumstances multiple players, or a player from second base, can score. If a runner moves up a base, but does not score, the hitter is not credited with a sacrifice fly. A sacrifice fly does not count as an at bat.
SH (Sacrifice Hit, Sacrifice Bunt)
A batter is credited with a sacrifice bunt when he bunts and successfully advances at least one runner at least one base, but he himself gets thrown out at first base (or would have if not for an error, as judged by the official scorer). A baserunner does not have to score, but can, for a hitter to get credit for a sacrifice bunt. A hitter who swings and advances a runner does not get credit for a sacrifice bunt. Sacrifice bunts do not count as at bats.
SHO (ShO, SH, Shutout)
A pitcher gets credit for a shutout when they pitch a complete game and allow zero runs. Usually this is accomplished by pitching a full nine innings, but in the event that the game is tied at zero at that point, the pitcher must pitch the whole game and not just nine innings in order to get credit for a shutout.
SLG (Slugging, Slugging Percent, Slugging Percentage)
Formula: ( TB/AB )
SLG is the most commonly used statistic used to measure a player's ability to hit for power. It is simply Total Bases divided by At Bats.
SV (S, Save)
A save is awarded to a pitcher who enters the game in a save situation and finishes the game with his team still in the lead. A save situation occurs when: the pitcher enters the game at or before the start of the final inning with his team winning by three runs or fewer; the pitcher enters the game at any point with the tying run on deck; or the pitcher pitches the final three innings of the game or more, regardless of the score. If a pitcher enters the game in a save situation, as long as he finishes the game he earns a save regardless of how many more runs his team scores after he enters.
Swing% (Swing Percent)
Swing Percent is the percent of total pitches that a hitter swings at.
SwStr% (Swinging Strike Percent)
Swinging Strike Percent is the percent of time the batter misses the ball when he swings. Foul balls do not count as swinging strikes.
TB (Total Bases)
Formula: ( H + 2B + (2*3B) + (3*HR)) or
((1*1B) + (2*2B) + (3*3B) + (4*HR))
Total bases is the number of bases a batter accumulates through his own hitting that's used primarily to calculate slugging percent. That's probably unnecessarily confusing. A batter gets credit for 1 base on a single, 2 bases on a double, 3 bases on a triple, and 4 bases on a home run.
TBF (BF, Batters Faced, Total Batters Faced)
Batters faced is a pitching statistic that is equivalent to plate appearances for a hitter. Rather than use Opp PA, TBF is used to denote the number of hitters that batted against a pitcher.
TC (CH, Chances, Total Chances)
Formula: ( A + PO + E )
Total chances is the number of times a fielder either recorded an out or assisted on an out plus the number of errors that fielder committed. The name is misleading, however, since situations where a fielder handles a ball but neither makes an out nor makes an error are not counted as a chance.
UZR (Ultimate Zone Rating)
Ultimate Zone Rating is an advanced defensive statistic designed by Mitchel Lichtman that is intended to rate a fielder's performance. The exact calculation is quite complex, as it involves analyzing the initial velocity and angle of the each ball in play and comparing the result of that play and the result of similar plays in the past. The end number is supposed to be a catch-all that more accurately rates defense than Range Factor or Fielding Percent. A UZR of zero is average, negative is below average, and positive is above average. It is controversial and even its proponents admit it takes a large sample size for the data to become reliable.
UZR/150 (Ultimate Zone Rating per 150)
UZR/150 is simply UZR per 150 games. Rating it per 150 games makes it easier to compare players with different amounts of innings and games played.
A pitcher gets credit for a win when he is the most recent pitcher to have pitched when his team takes the lead and holds onto the lead for the rest of the game. One exception is that a starting pitcher must go at least five innings to earn a win. If the starting pitcher's team is already ahead when he departs prior to the fifth inning, and they lead for the remainder of the game, the next pitcher to enter the game gets the win. Wins are a widely used stat for starting pitchers, but many people surprisingly do not realize a pitcher has no control over what his team does on offense, and thus that wins are a poor measure of a pitcher's performance.
WAR (Wins Above Replacement)
Wins Above Replacement is a one-number summary stat that gives a picture of how much value a player has provided to his team. For position players it evaluates a player's fielding, baserunning, and batting, compares it to the average for his position, and gives a number that represents how many wins a player contributed to his team above a replacement level player. For a pitcher, it does the same thing with his pitching statistics. A replacement level player is defined as a player who is readily available to every team for the league minimum, such as many AAA players. WAR is controversial because of the valuations it gives to various aspects of a player's performance, and because fielding and baserunning are difficult and unreliable statistics to measure. Its proponents admit it is not a be-all, end-all though, but rather a good launching point for discussing how good one player is relative to others.
WHIP (Walks Plus Hits Per Inning Pitched)
Formula: ( (BB + H) / IP )
After ERA, WHIP is probably the most commonly used derivative statistic used for pitchers. It evaluates a pitcher based on how many baserunners he allows, defining a baserunner as a walk or hit allowed. It ignores both what types of hits are allowed (singles or home runs?) and hit batters altogether, both of which make it a problematic, though certainly not useless, statistic.
wOBA (Weighted On Base Average)
Weighted On Base Average combines SLG and OBP into one statistic, much like OPS. Unlike OPS, however, wOBA does not weigh both component statistic equally, as a result of research that shows OBP is more valuable to a team than SLG. The resulting number is then scaled to resemble OBP (i.e. the league average wOBA is the same as the league average OBP for a given year).
WP (Wild Pitch)
A wild pitch occurs when a pitcher throws a ball that his catcher cannot catch or block so that at least one baserunner moves up one or more bases. If no baserunners move up, a passed ball cannot occur. The distinction between a passed ball and a wild pitch is that a wild pitch is deemed--by the official scorer--to be a pitch that was wild enough that it was the pitcher's fault the ball got away from the catcher.
WPA (Win Probability Added)
Win Probability Added is a statistic that evaluates a player's performance based on the context in which it comes. A home run when a player's team is winning by 12 runs is worth very little, whereas an infield single that scores the winning run with two outs in the bottom of the ninth is worth quite a bit. Statistically speaking, WPA is simply the change in the percentage that the batter's team will win from when the moment his plate appearance starts until when it ends. This percentage is calculated from years and years of results of baseball games, and does not take into account the other players in the lineup, the opposing pitcher, or the fielders. Batters can also be penalized or rewarded for baserunning plays made during their at bat, such as stolen bases. In the end, WPA is a fun and interesting statistic, but, given that research shows batting performance does not change based on context over the long haul, it is not very useful in predicting performance.
wRC (Weighted Runs Created)
Weighted Runs Created is based off of wOBA. Rather than an average, though, it presents a number that represents how many runs a player was worth to his team.
wRC+ is simply Weighted Runs Created adapted to a different scale and adjusted for the league, park, and year in which a hitter played. This makes it more useful for comparing seasons from different eras or players with different amounts of playing time. A wRC+ of 100 is average, while each point above or below 100 represents a 1 percent improvement or decline, respectively.
XBH (Extra Base Hits)
An extra base hit is scored any time a batter hits a double, triple, or home run. Extra base hits are indicative of a player's power, and are crucial in determining both ISO and SLG, the two main statistics used to measure that attribute.
xFIP (Expected Fielding Independent Pitching)
Expected Fielding Independent Pitching is FIP adjusted for a league-average home run rate (HR/FB). This is controversial, as some people believe pitchers have an ability to limit home runs, whereas some believe that any HR/FB not near league average is the result of lack, either good or bad. xFIP, like FIP, is on the scale of ERA, and is meant to discount both luck and defense in determining what a pitcher's ERA will likely be in the future.
X W-L (Expected Win-Loss, Pythagorean Win-Loss)
Expected Win-Loss is a team statistic, created by Bill James, that evaluates what a team's record should be by looking at the number of runs the team has scored compared to the number it has allowed. Based on research, over the course of season a team rarely is able to significantly outplay their Pythagorean Win-Loss and vice versa (though not never).
Z-Contact% (In Strike Zone Contact Percent)
Z-Contact % measures how often a batter makes contact with a pitch in the strike zone, given that he swings at the pitch. Foul balls count as making contact.
Zone% (Pitches in Strike Zone Percent)
Zone % measures the percentage of pitches a pitcher throws in the strike zone. Swinging strikes on pitches outside of the strike zone do not count as being in the strike zone.
Z-Swing% (In Strike Zone Swing Percent)
Z-Swing % measures the percentage of pitches in the strike zone that a batter swings at.