Analytics Glossary

The portion of spin that directly affects pitch movement, as opposed to gyroscopic (non-useful) spin.

The angle at which a pitch crosses the plate vertically, affecting how hitters perceive pitch location.

Estimated salary for arbitration-eligible players based on service time and performance.

Statcast metric measuring the average velocity of throws by a fielder, measured in miles per hour (mph).

A plate appearance that results in a hit, error, fielder's choice, or out, excluding walks, hit-by-pitches, sacrifice bunts, sacrifice flies, and catcher interference.

Average of available projection systems, often more accurate than individual projections.

Area Under the Receiver Operating Characteristic Curve, measuring classification model performance.

Dollar value assigned to players in fantasy baseball auction drafts.

The mean speed at which a batter hits the ball as measured by Statcast, excluding topped or weakly hit balls, typically above 60 mph.

Mean vertical angle of all batted balls for a hitter, indicating approach (fly ball vs ground ball tendency).

The difference between actual batting average and expected batting average.

The batting average on balls put into play, calculated as (H - HR) / (AB - K - HR + SF). League average is typically around .300.

A Statcast classification for batted balls with the ideal combination of exit velocity and launch angle, generally 98+ mph exit velocity with launch angles between 26-30 degrees, with the range expanding as exit velocity increases.

Percentage of batted ball events classified as barrels, representing the best possible contact outcomes.

Runs added or lost due to baserunning ability, including stolen bases, caught stealing, and taking extra bases.

MLB's official Statcast data website, providing access to pitch-level tracking data from 2015-present.

The speed of the bat at the point of contact or maximum speed during the swing, measured in miles per hour (mph). Elite bat speeds exceed 75 mph.

Component of WAR measuring runs created or lost from hitting, relative to average.

Statistical approach updating beliefs based on prior information and new data.

Number of walks a pitcher allows per nine innings, calculated as (BB × 9) / IP.

Walks per plate appearance, calculated as BB / PA × 100. League average is approximately 8-9%.

Batted balls with both high exit velocity (95+ mph) and optimal launch angle (10-30 degrees), indicating well-struck line drives.

The success rate at which a stolen base attempt has neutral expected value, typically around 70-75%.

Total WAR accumulated over an entire career.

Statcast metric estimating the likelihood of a fielder making a catch based on distance needed to travel, time available, and direction.

Runs saved by a catcher blocking pitches in the dirt.

Runs saved by a catcher through pitch framing, turning balls into strikes.

Percentage of batted balls hit to center field.

ERA estimator using individual component rates (K, BB, HR, etc.) rather than actual runs.

Percentage of pitches outside the strike zone at which a batter swings, calculated as swings outside zone / pitches outside zone.

The difference between the odds when a bet was placed and the closing line, indicating sharp betting.

A FanGraphs metric measuring how much better or worse a player performed in high-leverage situations.

Proportion of variance explained by a model, ranging from 0 to 1.

A swing on a pitch that could reasonably be put in play, used to measure swing decisions.

Range of values within which the true parameter likely falls, typically 95%.

Percentage of swings on which a batter makes contact, calculated as (Swings - Whiffs) / Swings.

Total expected value of a player contract based on projected performance.

Measure of linear relationship strength between two variables, ranging from -1 to 1.

Technique for assessing model performance by training and testing on different data subsets.

Percentage of pitches resulting in either a called strike or swinging strike, measuring pure stuff effectiveness.

A fastball variant that breaks slightly toward the pitcher's glove side, combining characteristics of a fastball and slider.

A defensive metric quantifying runs prevented or allowed compared to an average fielder at the same position, used in Baseball Reference's WAR calculation.

Current market value of one win above replacement, typically $8-10 million.

Runs saved or cost by turning (or not turning) double plays.

Baseball Prospectus's proprietary pitching metric that estimates a pitcher's true run prevention ability based on batted ball and pitch characteristics.

The magnitude of a difference or relationship, independent of sample size.

Combining multiple models to improve prediction accuracy (random forest, boosting, etc.).

ERA scaled so that 100 is league average, with lower being better. Calculated as (ERA / lgERA) × 100.

Runs saved or cost by avoiding or committing errors.

Statcast metric measuring the time between when a fielder catches the ball and releases it on a throw, measured in seconds.

The speed at which the ball leaves the bat after contact, measured in miles per hour by Statcast. League average is approximately 88-89 mph.

Statcast metric estimating batting average based on exit velocity and launch angle of batted balls, removing the influence of luck and defense.

The long-term average value of a bet, calculated as (Prob × Win) - ((1-Prob) × Loss).

Statcast metric estimating wOBA based on quality of contact (exit velocity and launch angle) rather than actual outcomes.

Percentage of swings with bat speed above a threshold (typically 70+ mph), indicating power potential.

Percentage of batted balls classified as fly balls, calculated as FB / Balls in Play.

Measure of how much each input variable contributes to model predictions.

FIP scaled so that 100 is league average, with lower being better. Calculated as (FIP / lgFIP) × 100.

Percentage of plate appearances starting with a strike.

The standard fastball grip with all four seams rotating, typically the pitcher's highest velocity pitch with backspin-induced rise.

Percentage of batted balls classified as ground balls, calculated as GB / Balls in Play. League average is approximately 45%.

A double play in which the batter hits a ground ball resulting in two outs.

Average leverage index when a player entered games.

Spin that rotates around the direction of travel (like a football spiral), providing little movement but affecting perception.

Betting half the Kelly-optimal amount for risk reduction.

The time a fly ball spends in the air from contact to landing or catch, measured in seconds.

Percentage of batted balls classified as hard contact based on exit velocity, typically 95+ mph. League average is approximately 35-40%.

Percentage of fly balls that become home runs. League average is typically 12-14%.

Horizontal movement of a pitch from the pitcher's perspective, measured in inches.

Number of home runs a pitcher allows per nine innings, calculated as (HR × 9) / IP.

Percentage of fly balls that are infield pop-ups, calculated as IFFB / FB.

The probability implied by betting odds, calculated as 1 / decimal odds.

Vertical movement of a pitch relative to a theoretical spinless pitch, accounting for gravity.

The number of outs recorded by a pitcher divided by three, with partial innings represented as .1 (1 out) or .2 (2 outs).

A measure of raw power calculated as SLG - AVG, or (2B + 2×3B + 3×HR) / AB. League average is approximately .145.

Hall of Fame metric averaging career WAR with 7-year peak WAR, compared to position average.

The initial burst of speed and direction by an outfielder when a ball is hit, measured in feet per second or reaction time.

Number of strikeouts a pitcher records per nine innings, calculated as (K × 9) / IP.

Ratio of strikeouts to walks, calculated as K / BB. League average for pitchers is approximately 2.5-3.0.

Strikeouts per plate appearance, calculated as K / PA × 100. League average for batters is approximately 22-23%, for pitchers approximately 22%.

Optimal bet sizing formula maximizing long-term growth: (bp - q) / b, where b = odds, p = probability, q = 1-p.

ERA estimator based solely on strikeout and walk rates, useful as a quick-and-dirty ERA predictor.

The vertical angle at which the ball leaves the bat, measured in degrees. Optimal angles for home runs are typically 25-35 degrees.

See Exit Velocity.

Percentage of batted balls classified as line drives, calculated as LD / Balls in Play. League average is approximately 20%.

The distance a baserunner takes from the base before a pitch is delivered, measured in feet.

Measure of game situation importance, with 1.0 being average. High-leverage situations have LI > 1.5.

A system for assigning run values to different offensive events (singles, doubles, walks, etc.) based on their impact on run scoring.

Percentage of baserunners who do not score, calculated as (H + BB + HBP - R) / (H + BB + HBP - 1.4×HR). League average for pitchers is approximately 72%.

Model-based metric measuring the quality of pitch location relative to optimal targets.

Measure of prediction accuracy for probability estimates, penalizing confident wrong predictions heavily.

Average absolute difference between predictions and actual values.

Relief appearance that decreases team's win probability by at least 6%.

The unique identifier assigned to players by MLB Advanced Media, used in Statcast and most modern baseball databases.

Park factor calculated over multiple seasons (typically 3-5) to increase stability and reduce single-season variance.

True implied probability after removing bookmaker's margin.

Percentage of pitches outside the strike zone at which a batter swings. League average is approximately 30%.

Statcast's primary defensive metric, quantifying runs prevented above an average fielder based on catch probability and successful plays.

Frequency with which a batter reaches base, calculated as (H + BB + HBP) / (AB + BB + HBP + SF). League average is approximately .320.

Total offensive value combining batting runs and baserunning runs in WAR calculations.

Percentage of batted balls hit to the opposite field.

OPS scaled and adjusted for park factors so that 100 is league average, with higher being better.

Arrangement of batting order to maximize expected runs, often placing highest OBP at top.

Runs saved by an outfielder's arm preventing runners from advancing.

When a model learns noise in training data, performing poorly on new data.

A metric quantifying how much a ballpark influences run scoring, with 100 being neutral, >100 favoring hitters, and <100 favoring pitchers.

Baseball Prospectus metric estimating ERA based on underlying pitch characteristics.

The highest WAR total achieved over a player's best consecutive seasons (typically 7 years).

Baseball Prospectus's Player Empirical Comparison and Optimization Test Algorithm, a projection system.

The effective velocity of a pitch accounting for release extension, giving the batter's perceived speed at release.

The skill of a catcher in receiving pitches to influence umpire strike calls, typically measured in runs added or strikes gained.

Run value of each pitch type thrown by a pitcher, based on outcomes and context.

Wins component attributed to pitching in win probability models.

Component of WAR measuring runs prevented or allowed from pitching, relative to average.

Combined metric incorporating Stuff+, Location+, and pitch sequencing.

Total number of completed batting appearances, calculated as AB + BB + HBP + SF + SH.

Composite metric combining chase rate, zone swing rate, and contact rates to measure a hitter's approach quality.

Performance difference when facing same-handed vs opposite-handed pitchers/batters.

Average leverage index for all situations faced by a pitcher.

Time from when the pitch hits the catcher's mitt to when the throw reaches second base on a steal attempt, measured in seconds.

Run value adjustment in WAR based on defensive difficulty of position played.

Updated belief about a parameter after incorporating data.

Whether an effect is large enough to matter in real-world terms.

Initial belief about a parameter before seeing data in Bayesian analysis.

Forecasted WAR for upcoming season based on projection systems.

Bets from recreational bettors, often favoring favorites and overs.

Percentage of batted balls hit to the pull field (left field for right-handed batters, right field for left-handed batters).

Percentage of two-strike pitches that result in a strikeout.

Statistical measure indicating the proportion of variance in the dependent variable explained by the independent variable(s), ranging from 0 to 1.

The number of runs a player contributes above a league-average player, before adjustments for position and replacement level.

Runs saved or cost by a fielder's range, measuring ability to get to balls.

The number of runs a player contributes above a replacement-level player, the foundation of WAR calculations.

Runs above average based on the 24 base-out states, measuring situational production.

Statistical technique for modeling the relationship between variables, or the tendency for extreme performances to move toward the mean over time.

The tendency for extreme performances to move toward average over time.

The expected performance level of a player who could be readily acquired from the minor leagues or free agency, approximately 20 runs below average per 600 PA.

Net profit divided by total amount wagered, expressed as percentage.

RE24 converted to wins using runs per win.

Standard deviation of prediction errors, measuring model accuracy.

Percentage comparing the actual distance traveled by an outfielder to the optimal straight-line route to catch location.

The average number of runs expected to score in an inning given a specific base-out state.

Table showing expected runs for all 24 base-out states.

The change in run expectancy caused by a specific event, used in linear weights calculations.

Number of observations needed for statistical stability, varying by metric.

Movement generated by the orientation of seams disrupting airflow asymmetrically, independent of Magnus force.

Additional distance gained by a baserunner after the pitch is delivered but before a play is made.

Bets from professional or highly-informed bettors that move lines.

Estimate that pulls extreme observations toward the population mean.

Relief appearance that increases team's win probability by at least 6%.

An ERA estimator that accounts for strikeouts, walks, ground balls, and their interactions, designed to predict future ERA better than FIP.

A fastball variant with arm-side movement and downward break, designed to induce ground balls.

Total bases per at-bat, calculated as (1B + 2×2B + 3×3B + 4×HR) / AB or TB / AB. League average is approximately .400.

A breaking pitch with tight spin and late break, typically breaking glove-side and down from the pitcher's perspective.

The direction of a pitch's spin represented as a clock face from the catcher's perspective, measured in degrees from 0-360.

Percentage of a pitch's total spin that contributes to movement, excluding gyroscopic spin.

The rate at which a baseball rotates while in flight, measured in revolutions per minute (rpm). Higher spin rates typically create more movement.

The horizontal angle of a batted ball relative to center field, measured in degrees.

Statcast metric measuring a player's feet-per-second rate on competitive plays, such as home-to-first on ground balls. League average is approximately 27 ft/s.

Percentage of batted balls where the ball is hit near the sweet spot of the bat, resulting in optimal energy transfer.

The sample size at which a metric is approximately 50% skill and 50% noise.

Measure of the precision of an estimate, decreasing with sample size.

Fantasy metric measuring value of statistics toward category standings.

MLB's advanced tracking technology that measures player movements, pitch characteristics, and batted ball data using high-resolution cameras and radar.

The probability that an observed result occurred by chance, typically p < 0.05.

Projection system emphasizing recent performance and regression.

Run value of stolen base attempts, calculated as (SB × runSB) + (CS × runCS).

Model-based metric predicting strikeout rate based purely on pitch characteristics (velocity, movement, etc.).

The difference between a player's projected production value and their salary cost.

Percentage of batted balls with launch angles between 8-32 degrees, optimal for base hits.

The distance the bat travels during a swing from start to contact, measured in feet. Shorter swings typically correlate with better contact rates.

Percentage of all pitches that result in swinging strikes, calculated as swinging strikes / pitches. League average is approximately 11%.

ERA estimator from StatCorner accounting for batted ball types and sequencing.

The degradation in pitcher performance each time through the batting order, typically 0.3-0.4 runs per time through.

Sum of bases from all hits, calculated as 1B + 2×2B + 3×3B + 4×HR.

The location where two different pitch types are visually indistinguishable to the batter, typically 20-25 feet from home plate.

Fastball grip showing two seams in rotation, typically with more arm-side movement and slightly lower velocity than a four-seamer.

Metric quantifying baserunning value from all non-stolen base situations (taking extra bases, tagging up, avoiding outs), measured in runs.

Baserunning value from non-stolen base events: advancing on hits, sac flies, wild pitches, etc.

Percentage of total pitches that are a specific pitch type.

Defensive metric dividing the field into zones and measuring a fielder's effectiveness in each zone compared to the league average, measured in runs.

The angle at which a pitch crosses the plate relative to the ground, measured in degrees. Flatter angles (closer to 0) are generally harder to hit.

The bookmaker's commission built into betting odds, typically 4-5% for MLB.

A player's production above replacement level in fantasy context.

A comprehensive metric estimating the total number of wins a player contributes above a replacement-level player, combining batting, baserunning, fielding, and positional value.

Sum of a player's seven highest single-season WAR totals.

Baseball Prospectus's version of WAR, using different component metrics and methodologies.

Run value above average for all curveballs thrown.

Run value above average for all changeups thrown.

Run value above average for all fastballs thrown, per 100 pitches scaled.

Run cost of grounding into double plays relative to opportunity.

Percentage of swings that result in misses, calculated as whiffs / swings.

Number of baserunners allowed per inning, calculated as (BB + H) / IP. League average is approximately 1.30.

Probability of winning given current game state (inning, score, base-out state).

The change in win probability caused by a specific event, summed to show a player's contribution to team wins.

An advanced batting metric that weights different offensive events (singles, walks, home runs, etc.) by their actual run value, scaled to OBP. League average is approximately .320.

The constant used to convert wOBA to runs, typically between 1.15-1.25, varying by season based on run environment.

The difference between actual wOBA and expected wOBA, indicating luck or over/underperformance.

Win Probability Added divided by average leverage, showing performance independent of situation.

Runs created above average based on wOBA, calculated as ((wOBA - lgwOBA) / wOBA_scale) × PA.

wRC scaled so that 100 is league average and adjusted for park factors, with higher being better. Accounts for both league and park context.

Runs added from stolen bases, calculated as (SB × 0.2) - (CS × 0.43), using linear weights for SB success and failure.

Run value above average for all sliders thrown.

Statcast metric estimating what a player's batting average should be based on exit velocity and launch angle, removing luck and defense.

ERA estimator based on expected outcomes from quality of contact metrics like xwOBA, xBA, and barrel rate.

FIP calculated using league average HR/FB rate instead of actual HR/FB, designed to remove luck from home run outcomes.

Expected ISO based on batted ball quality, excluding luck and defense.

Statcast metric estimating slugging percentage based on batted ball quality (exit velocity and launch angle).

Statcast metric estimating wOBA based on exit velocity and launch angle rather than actual outcomes, designed to measure quality of contact independent of luck.

xwOBA calculated only on contacted balls, excluding strikeouts and walks, isolating pure batted ball quality.

How well a metric predicts itself from one season to the next.

Percentage of swings on pitches in the zone that result in contact. League average is approximately 85%.

Percentage of pitches in the strike zone at which a batter swings. League average is approximately 68%.

Dan Szymborski's projection system using player comparisons and aging curves.

Percentage of pitches thrown inside the strike zone, calculated as pitches in zone / total pitches. League average is approximately 45%.