We will continually populate this page with a glossary of commonly used statistical terms and frameworks, as well as links to other great sites and blogs.


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Baseball Reference


An acronym for batting average on balls in play, BABIP measures how many of a player’s batted balls end up as hits. To differentiate from batting average, BABIP removes strikeouts and home runs from the equation, as those at bats do not involve balls in play. League average BABIP tends to fluctuate between .290 and .310, and batters who demonstrate a BABIP significantly higher or lower than this average are generally considered to be candidates for regression to the mean, as along with defensive prowess, luck is a key component of BABIP. Studies have shown that some batters (particularly those with speed and/or high skill level) are able to maintain above-average BABIP over a protracted period. However, similar research has shown that a pitcher’s ability to control BABIP is very limited. Luck and defense are the primary contributors, and as such, sabermetric thinkers tend to exclude BABIP when evaluating/projecting pitchers, and instead focus on so-called three true outcomes: walks, strikeouts, and home runs surrendered.

An indexed representation of a pitcher’s ERA relative to that of the league as a whole, with adjustments made for park factors. Calculated by multiplying 100 by (lgERA/ERA); 100 is considered league average. Note that a pitcher with an ERA lower than league average will have an ERA+ greater than 100, meaning a higher ERA+ is better than a lower one.

Fielding independent pitching attempts to measure a what a pitcher’s ERA might have been had batted balls resulted in an average number of hits (see: BABIP). It takes into consideration, in varying weights, home runs, walks, strikeouts, and batters hit by pitch and is scaled to roughly match standard ERA ranges. (See also: xFIP)

OPS is simply the sum of a batter’s on base and slugging percentages. While imperfect in the sense that it weights the two measures in equal importance (when OBP has a greater correlation to run scoring vs. SLG), OPS is a quick-and-easy to digest measure of offensive ability. It reflects both a player’s ability to reach base, and hit with power. Since 2010, the average MLB player has posted a .725 OPS, based on a rough averages of .325 OBP and .400 SLG.

An indexed version of OPS, OPS+ is scaled such that a league average is always 100. It is adjusted for park and league scoring levels, making it possible to compare players across different teams and years. One important note: because of the additive nature of OPS, a player with both an OBP and SLG each 50% better than league average will have an OPS+ of 200 (twice the league average OPS+) while still having an OPS that is actually only 50% better than the average OPS of the league.

UZR – Ultimate Zone Rating
UZR is a measure of defensive performance that attempts to quantify how many runs a player saved (or cost) his team. The formula takes into account runs saved by outfielders’ throwing, infielders’ double plays and both range and errors for all fielders, and is park-adjusted. Since UZR is a counting statistic, the magnitude of the statistic will tend to increase with playing time. UZR/150 standardizes the statistic over 150 games played. Small sample sizes (generally considered less than 50 innings) can be highly volatile and three years of data at a position is generally accepted to be the period over which the data is considered reliable.

WAR  - Wins Above Replacement
A single number that presents the number of wins the player added to a team above what a replacement player (think AAA or AAAA) would have produced. Calculated by tracking how much a player’s actions increased or decreased the likelihood that runs would be scored using historical probabilities. This value includes defensive support and includes additional value for high leverage situations.
Scale: 8+ MVP Quality, 5+ All-Star Quality, 2+ Starter, 0-2 Reserve, < 0 Replacement Level
Sometimes referred to as rWAR to differentiate from fWAR, the former was developed by Sean Smith and is used by, while the latter is Fangraphs’ version of the same. The formulas differ slightly and fWAR values tend to be slightly greater in magnitude.

Similar to FIP, expected fielding independent pitching is a regressed version of FIP that replaces actual home runs surrendered with an estimate of how many a pitcher might have been allowed by using a league average home run-flyball ratio. It’s designed to eliminate more chance than FIP, because xFIP is based on the assumption that HR-FB ratios are not pitcher dependent. However, it may prove less accurate in cases in which a specific pitcher does have a higher propensity to surrender home runs on fly balls.