I decided to put this glossary together as a resource for people who may come to this site, see me use a stat, and think “what does that mean?”
Overall, I try to keep the stats I use pretty basic, but a lot of them are still not familiar stats to some readers, and most are not used in a typical box score.
Before we start, I’d also like to direct readers to the glossary at FanGraphs, which explains not only the stats I use, but also a lot of more advanced stats that you may see cited elsewhere.
Now, onto the terms:
On-base percentage (OBP): Basically what batting average should be, but isn’t. On-base percentage is figured by taking a player’s hits, walks and times hit by a pitch, adding them together, and dividing them by the number of plate appearances that player has made. This is widely considered to be the single most important statistic in baseball.
Plate appearances (PA): A part of the OBP formula, plate appearances are different from at-bats. Like OBP to batting average, they’re what at-bats should be but aren’t. A plate appearance is simply the total number of times a player has come up to bat. This differs from at-bats in the way that walks and sacrifices are not counted as official at-bats, but are counted as plate appearances.
Slugging percentage (SLG): Not really a percentage, but a good measure of a player’s power anyway. The formula is total bases divided by at bats.
Triple-slash stats: Simply batting average/on-base percentage/slugging percentage. So if you see me write .300/.371/.471, it means a player is hitting .300 with a .371 OBP and a .471 SLG.
On-base plus slugging (OPS): Exactly what it sounds like: take a player’s on-base percentage and their slugging percentage and add them together. It’s a quick-and-dirty way of evaluating overall offensive prowess. In general, you want a player’s OPS to be .800 or better.
Isolated power (ISO): Slugging percentage – batting average. Basically the rate at which a player hits for extra bases. An ISO above .200 is good, above .250 is very good, and any higher than that is what I can only describe as Albert Pujols good.
K/BB ratio: For pitchers, the number of batters they strike out (K) vs the number of batters they walk (BB). An important statistic because they’re the only areas a pitcher has full control over. Once a ball is put into play, it’s up to the defense to turn it into an out. For hitters, the number of times they strike out at the plate (K) vs the number of times they walk (BB). This gives you a good idea of a player’s plate discipline.
K/9, BB/9, H/9: Respectively, strikeouts per nine innings, walks per nine innings and hits per nine innings. I’ll rarely cite H/9, but I thought I should include it in the glossary anyway.
Fielder independent pitching (FIP): In a nutshell, what a player’s ERA would look like if everything worked independently of the fielders. Focuses on strikeouts, walks and home runs, adjusts for the average ERA for a given league and season, and spits out a final result that looks exactly like ERA. Thus, an FIP around 4 is average, around 3 is very good, and around 5 is bad. If a player has a much better or much worse ERA than their FIP, expect the two to come closer to each other.
Expected FIP (xFIP): FIP plus an adjustment for how many fly balls a pitcher gives up. The idea behind this is that home runs are a product of fly balls, and what may be a flyout in one park is a home run in another part. In theory it’s a better predictor of a pitcher’s future ERA than simple FIP. I’m skeptical, but I also see the use of the stat.
Batting average on balls in play (BABIP): A player’s batting average on balls put into the field of play. Thus, strikeouts, foul balls, walks and home runs do not count for this statistic, because those balls aren’t in play. The average BABIP is around .300, though it can be consistently higher for hitters who hit a lot of line drives, players who run fast enough to beat more throws than average to first base, etc. It can also be consistently lower for hitters who aren’t gifted with any of those abilities. Those hitters are usually either pure power hitters like Adam Dunn or bench warmers. For pitchers, the statistic can give you an idea of if his current performance can stay on pace. So if a guy is striking out about the same number of batters as he always does, walking about the same number of batters as he always does, and giving up about the same amount of home runs as he always does, yet he’s performing way better or way worse than he always does, check his BABIP. He’s probably having luck (or a way better or way worse defense) affect his results.
Walks + hits divided by innings pitched (WHIP): Exactly what it sounds like. A quick way of seeing how many batters are reaching base against a pitcher.
Replacement player: The performance you would expect from any scrub called up from AAA. Replacement level players generally perform well below average as compared to average Major League players. If your team is starting a player who is performing at or below replacement level on an every-day basis, your team needs to upgrade that position. These players are mentioned in this glossary as it’s important to understand what they are in order to understand the value of the next few statistics.
Ultimate zone rating (UZR): The best defensive statistic available to the general public at the moment. In a nutshell, UZR determines how many runs a player saves defensively above a replacement-level player. This is done by dividing the field into certain zones, and making each player responsible for defending a specific zone. UZR then factors in how many runs they’ve saved based on their ability to get to the ball, the ability to convert the ball they get to into an out, and even the ability to help turn a double play. Usually it takes a much longer time to evaluate a player’s defensive ability than their offensive ability, because a player doesn’t get as many defensive chances per season as they do plate appearances. Thus, an overall picture of a player’s defensive prowess measured by UZR is unlikely to carry an validity until they’ve played around three full seasons.
UZR/150: UZR divided by 150 games played.
Wins above replacement (WAR): An overall assessment of how many more wins a given player contributed to their team than a replacement-level player at the same position would. WAR considers both offense and defense. To take two shortstops as an example. even though Derek Jeter has a horrible career UZR, he’s still far better than a replacement level player because he hits so well at a position that usually doesn’t see much offense. Even though Jack Wilson is a bad hitter, he too is better than a replacement level player because he plays great defense at a key defensive position. The average Major League player is worth about 2.5 wins more than a replacement-level player, and the best players are worth 5 or more wins above replacement level.
