Sabermetrics
Sabermetricsis the specialized analysis of baseball through objective, empirical evidence, specifically baseball statistics that measure ingame activity. The term is derived from the acronym SABR, which stands for the Society for American Baseball Research. It was coined by Bill James, who is one of its pioneers and is often considered its most prominent advocate and public face. 
BA: Batting Average 
... (H/AB) 
The rate at which a player gets a hit relative to his total atbats. The average BA in the Majors for 2011 was .255. 
BABIP: Batting Average on Balls in Play 
... (H  HR)/(AB  K  HR + SF) 
How many of a player's balls in play go for hits. BABIP removes HR, BB, and K  outcomes not impacted by defense  from the player's average. It can serve as a rough estimate of a player's luck, and can help predict future performance. The league rate stays around .300 (it was .295 in 2011). A higher BABIP often means a player is on the right side of luck (hits are falling in) and will regress. A lower number often implies a player is on the wrong side of luck and will improve. The metric can be used mostly for pitchers, and somewhat for hitters. 
BB%: Walk Percentage 
... (BB/PA) 
How often a player walks relative to his total plate appearances. The average walk rate across MLB in 2011 was 8.1%. Jose Bautista walked at a rate of 20.2% in 2011; Yuniesky Betancourt, 2.7%. 
BB/9: Walks Per 9 Innings Pitched 
... (BB/IP)*9 
How many batters a pitcher walks per nine innings. An excellent BB/9 is below 2. The average BB/9 across MLB in 2011 was 3.1. (Roy Halladay had a BB/9 of 1.35 for 2011.) 
K/9: Strikeouts Per 9 Innings Pitched 
... (K/IP)*9 
How many batters a pitcher strikes out per every 9 innings. An excellent K/9 is 9.0, or a batter an inning. The average K/9 across MLB in 2011 was 7.1. 
K%: Strikeout Percentage 
... K/PA 
How often a batter strikes out relative to his total plate appearances. The average K% across MLB in 2011 was 18.6%. When Mark Reynolds set the singleseason strikeout record in 2009 with 223 whiffs, his rate was 33.7%. Placido Polanco's career K% entering 2012 is 8.4%. 
ERA: Earned Run Average 
... 9 * (Earned Runs/IP) 
The number of earned runs a pitcher allows per nine innings pitched  it does not include runs which scored because of errors or passed balls. The average ERA across MLB in 2011 was 3.94. 
ERA+: Adjusted ERA 
... (LgERA/ERA) * 100 
A pitcher's ERA relative to the leagueaverage ERA and adjusted for his home ballpark. 100 is league average, below 100 would be below average; over 100 is above average. A 150 ERA+ means the pitcher's ERA is 50% better than a leagueaverage pitcher. 
FIP: Fielding Independent Pitching 
... [(HR * 13) + (BB + HBP  IBB)*3  (K * 2)]/IP 
A single, ERAlike number, focusing on a pitcher's K rate, BB rate and HR rate (strikeouts, walks and home runs are not impacted by defense). By limiting its focus to factors a pitcher can control, FIP demonstrates a pitcher's effectiveness better than ERA. The average FIP across MLB in 2011 was 3.94. 
xFIP: Expected Fielding Independent Pitching 
...(FB * .106) * 13 + (BB + HBP  IBB) * 3  (K * 2))/IP 
Calculated much like FIP, but with the home run rate kept at the leagueaverage rate of 10.6%. (Individual pitchers' home run rates often vary year to year.) The average xFIP in MLB for 2011 was 3.94. 
FRS: Fielding Runs Saved 

The measure of runs saved beyond what a leagueaverage player at that position might have saved. The league average is 0. 
RC: Runs Created 
... (TB * (H + BB)) / (AB + BB) 
An estimation, developed by Bill James, of a player's offensive contribution to his team, measured in terms of runs. A precursor of wOBA (below). 
OBP: On Base Percentage 
... (H + BB + HBP)/(AB + BB + HBP + SF) 
The rate at which a player gets on base relative to his total plate appearances. (PAs include walks, hitbypitch, sacrifice hits and catcher interference; atbats don't.) The average onbase percentage in MLB for 2011 was .321. 
OPS: On Base Percentage Plus Slugging Percentage 
... (OBP + SLG) 
A single stat that estimates a player's offensive production. Note that in OPS, OBP and SLG are weighted evenly (they're summed), despite the two metrics being weighted differently themselves: OBP is calculated from plate appearances and SLG from atbats. 
OPS+: Adjusted OPS 
... 100 * ( OBP/lgOBP + SLG/lgSLG  1)/BPF 
A batter's OPS relative to the leagueaverage OPS and adjusted for his home ballpark. 100 is league average, anything below 100 is below average; over 100 is above average. Bautista held a 181 OPS+ in 2011, meaning his OPS was 81% better than a leagueaverage hitter. 
SLG: Slugging Percentage 
... ((1B) + (2 * 2B) + (3 * 3B) + (4 * HR))/AB 
The rate at which a player gains total bases relative to his total atbats. The average slugging percentage in MLB for 2011 was .399. 
UZR: Ultimate Zone Rating 

How many runs above or below average a player was worth defensively. 0.0 is considered average. Developed by Mitchel Lichtman. 
WAR: Wins Above Replacement 

A single statistic summarizing a player's offensive and defensive contributions. WAR calculates the amount of wins a player adds to his team compared to the contribution that would be expected from a replacementlevel player. A replacementlevel player is a player who can be signed for mimimum cost, i.e. someone on the waiver wire or a Minor League free agent. 0.0 WAR over a season denotes a replacementlevel player, 2.0 is a regular starter, 5.0 is AllStar worthy and a WAR of 8 or above means the player is worthy of MVP consideration. 
fWAR: 

Fangraph.com's WAR calculation, which includes FIP for pitchers, and UZR for defense (see above)

rWAR: 

BaseballReference.com's WAR calculation, which includes defensive runs saved (plusminus) when calculating pitching, TotalZone for defense, and baserunning

WARP: 

Wins above replacement player according to Baseball Prospectus

WHIP: Walks and Hits Per Innings Pitched 
... (W + H)/IP 
The number of walks and hits allowed by a pitcher per inning. Essentially, this equals baserunners per inning. A WHIP of 1 or below is considered elite. The average WHIP across MLB in 2011 was 1.32. 
wOBA: Weighted On Base Average 

A single stat that takes into account how a player gets on base, not just that a player gets on base, by assigning more value to extrabase hits. wOBA weighs each aspect of hitting in proportion to its run value, unlike OPS. The average wOBA across MLB in 2011 was .316. 
WPA: Win Probability Added 

A calculation of how a player affects his team's win expectancy from play to play, over the course of a game. For example, a player who homers in a lateandclose situation will have a higher WPA than one who strikes out. 


