Baseball Statistics and Acronyms Explained

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Baseball analysis is fraught with acronyms, abbreviations, numbers, and statistics.  This post is intended to be a cheat sheet and a reference, allowing the average PSAer to make more statistically informed posts, as well as decipher the cryptic messages of others.  I will keep a link to this post in my signature, and hopefully it will be helpful in our future discussions of Yankees baseball.

(Batting Average / On Base Percentage / Slugging Percentage)  The simplest and most basic way to gauge a batter's contribution is through the "triple slash" of batting average, on base percentage, and slugging percentage.  Batting average measures the rate at which a player records hits to making outs, on base percentage measures the rate at which a player reaches base to plate appearances, and slugging percentage measures the rate at which a player accumulates "total bases" to making outs.  While each of these statistics have shortcomings and can not be viewed in a vacuum, the triple slash is a simple and accurate way to gauge the strengths and weaknesses of a hitter.

(On Base Percentage Plus Slugging Percentage) OPS is the most basic composite stat.  It combines two abilities of a batter (the ability to get on base and the ability to hit for power) and combines them into one number, an indicator of overall value.  OPS, and the understanding of the importance of OBP and SLG, led to many of the subsequent breakthroughs in analysis, however the main shortcoming is in an equal weight being given to OBP and SLG.  While this issue can be addressed in other statistics, OPS is an excellent way to quickly judge the overall offensive contribution of a player.  OPS+ is a normalized version of OPS, with 100 being exactly league average.

(Isolated Power) ISO is a measure of a hitter's ability to record extra bases with a hit.  Thus, if a player records a hit, how many extra bases do they average?  Players with good speed and/or power will record high ISO values, and ISO is often referenced when discussing the "power potential" for a minor league prospect.

(Weighted On Base Average) wOBA assigns a linear weight to each of the possible outcomes of a plate appearance.  (In the above formula, NIBB = Non intentional walk, RBOE = Reached base on error)  The coefficients of each outcome are determined by their relative correlation to runs being scored as a result of that event.  wOBA is one of the most inclusive and revealing statistics for batting analysis.  (More)

(Weighted Runs Created) wRC+ uses the linear weights of wOBA to determine the runs created by a specific batter, and then is park and league adjusted, and normalized to 100.  In much the same way as OPS+ is a normalized version of OPS, wRC+ gives you a more detailed way to compare a batter's contribution to the league average.  (More)

(Batting Average on Balls in Play)  BABIP measures the rate at which batted balls land for hits in the field of play.  As any ball in play can result in anything from a triple play to an inside-the-park home run, there is a great variance in the result of batting a ball.  Also, as this is not a direct result of contact or skill, but merely the trajectory of the ball and the positioning of the defense, BABIP is often used as a measure of a player's luck.  BABIP in and of itself does not signal if a player is getting lucky or unlucky, or whether a level of play is or is not sustainable, however, when combined with contact ratios (LD%, GB%, FB%, HR/FB) these things can be determined with more certainty.  The general rule of thumb is that BABIP ~ LD% + .120 and a player's hitting chart can be used to estimate xBABIP (expected BABIP) as another way to judge ball in play luck.

(Earned Run Average) ERA is a measure of how many earned runs a pitcher gives up compared to how many outs their team is able to record with them on the mound.  Like a triple slash, ERA can be the simplest way to measure the overall effectiveness of a pitcher, however, the main shortcoming is the correlation between ERA and the ball park as well as the aptitude of the pitcher's defense.

(Walks and Hits per Innings Pitched) WHIP is a measure of how many baserunners a pitcher allows per inning pitched.  As the converse of OBP, it allows you to judge the ability of opposing batters to reach base.  Similar to OBP, the reason that WHIP shouldn't be used as a standalone metric is that it does not give adequate weight to extra base hits.

(Fielding Independent Pitching) As referenced earlier, one of the main shortcomings of ERA is in the play of the defense being outside the control of the pitcher.  As such, DIPS (Defense Independent Pitching Statistics) have been created, in order to isolate the pitcher's contribution to the prevention of runs.  As such, only plays which can not be affected by the defense are considered (home runs, walks, and strikeouts).  The modifier of 3.10 (sometimes 3.08 or 3.20 are used) is used to put FIP on the same scale as ERA, meaning that a pitcher with a 3.50 FIP would be expected to have a 3.50 ERA if he played with a league average defense.

(Expected Fielding Independent Pitching) xFIP accounts for differences in park size and luck in home run rates by standardizing the home run term.  On average, 10.6% of fly balls carry over the fence, and as such, xFIP is park and luck neutral.  As such, it can address some of the major concerns about the reliability and predictive nature of ERA.  (More)

(True ERA) tERA is an exciting statistic on the frontier of new possibilities of baseball analysis.  It uses the speed and trajectory of batted balls (using Hit F/X to track the flight of the ball) to determine the expected outcome of each ball in play.  This concept and technology is very new, but once all of the kinks get worked out, this could be a very exciting way to measure what a pitcher's ERA should be.  (More)

(Ultimate Zone Rating) UZR is a measure of a defensive players ability to make plays within certain areas of the field.  In order to calculate UZR, the field is split into 78 zones.  The player's ability to make plays in each of these zones in comparison to other fielders of the same position result in positive and negative run values.  UZR is said to need 3 years of data in order to have adequate sample size, and it's reliability does not extended to catcher or first base.  It can also be normalized to 150 innings with UZR/150, which gives a rate, instead of a counting stat.  While defense is the hardest to quantify and defensive metrics are the least predictive, UZR provides a sizeable improvement over fielding percentage, gold gloves, or the "eye test." (More and More)

(Wins Above Replacement) WAR is a measurement of how many extra wins a team would be expected to win as a result of a single player.  Each player is compared against a "replacement level" player, who is of the talent of the average AAA player or a cheap journeyman signing that is available to any team.  From the offensive and defensive contributions of a position player, or from the FIP of a pitcher, a number of runs can be assigned to an individual player.  These runs are then correlated to a number of wins.  (More)