Use of Statistics
Statistics are used for a lot of things. This page will attempt to explain these statistics.
Not all statistics try to measure the same thing the same way. Some statistics try to measure raw achievements while others use complicated formulas to try to measure players' value. Within the range of statistics, some common distinctions can be made.
Counting vs. Rate
The most obvious distinction is between statistics the measure a quantity of production and those that measure a rate of production.
A counting stat is one that measures a player's total production without reference to his number of opportunities. Common counting stats include hits, home runs, and innings pitched. While the most common counting stats are raw numbers, there are many sabermetric stats that are also expressed the same way, like runs created or VORP.
In contrast to counting stats, rate stats try to show a rate of productivity by dividing the number of successes (or failures) by the number of opportunities. Most rate statistics will include a term like "average" or "percentage" in their name as in indication that they are the result of a division process. Many of the most popular measures of ability are rate stats, including batting average and ERA.
It's important to note that many counting stats are given with an implicit number of opportunities, like a season or career. This kind of implicit number of opportunities may or may not balance out. While two starting pitchers will normally have about the same number of starts in a season and consequently equal opportunities to collect wins, the same is not true of a starting pitcher and a relief pitcher, so a comparison of their counting stats will not be valid.
Descriptive vs. Value
Another important distinction is between statistics that try to describe the way a player performs and ones that try to measure the value of that performance. A weak hitting, slick fielding shortstop is a very different player from a slow, power hitting first baseman, and that will tend to show up when using a descriptive statistic like ISO or stolen base percentage, which shows how well each player does at one aspect of the game. At the same time, they may score identically using a statistic like VORP, Wins Above Replacement or Win Shares that flattens their whole offensive and defensive contribution to the team into a single number.
Raw vs. Adjusted
Playing conditions have changed radically over baseball history. In the 1930 NL, for instance, the league as a whole (including pitchers) hit .303, while Carl Yastrzemski won the 1968 AL batting title by hitting .301. There are similar, though smaller, differences between players in the same season but different ballparks. Coors Field, for instance, is very batter friendly while Dodger Stadium is very pitcher friendly. Most statistics are raw statistics that make no attempt to adjust for these kinds of contextual factors.
Adjusted statistics are "normalized" by comparing them to the league average and adjusting for a park factor. Statistics like OPS+ and ERA+ are expressed as ratios of the player's productivity to league context, so that 100 represents league average and numbers larger than 100 are better. Other statistics, like EQA are expressed as performance in some "neutral" environment, i.e. a .260 batting average or 4.00 ERA is league average.
Batting average measures the rate at which a batter accumulates hits. It has often been used as an indicator of overall batting value, but it misses the value contributed by walks and extra base power.
On-Base percentage measures the rate at which a batter reaches base by hit or walk, thus correcting a weakness of batting average. The disadvantage of this is that it considers a walk to be just as valuable as a single.
Slugging percentage is similar to batting average but tries to measure power by giving batters extra credit for hitting doubles, triples, and home runs. Like batting average, it gives no credit for walks.
OPS is the sum of a player's on base and slugging percentages. It gives batters credit for getting hits, walking, and hitting for power, and is becoming popular as a rough measure of overall batting ability.
OPS Plus is a park and league adjusted version of OPS.
Walks are an indication of a player's patience and strike zone judgment.
Intentional Walks are often an indicator of how feared a player is as a batter; feared batters are walked intentionally to avoid letting them hit in critical situations.
Earned Run Average is a measure of the rate at which a batter allows the other team to score runs. It is a primary measure of a pitcher's value.
ERA Plus (ERA+) is a park and league adjusted version of ERA.
WHIP is a measure of the rate at which a pitcher allows batters to reach base. It makes no distinction between walks and hits, and is roughly the pitcher's equivalent to on base percentage.
K/9 measures the rate at which the pitcher strikes out batters, a key measure of pitching style.
BB/9 measures the rate at which the pitcher allowes walks, another measure of pitching style.
Stolen Bases (SB) count how often a batter steals bases. A high total is indicative of a fast player, but a low total may not indicate a slow one.
Caught Stealing (CS) measures how often the batter was caught stealing.
Stolen Base Percentage measures a runner's success rate when stealing bases.
Winning Percentage measures a team's overall success.
Pythagorean W-L is an attempt to predict how many games a team should have won based on its runs scored and allowed. A team whose Pythagorean W-L is much higher than its actual W-L was probably lucky and can't be counted on to keep winning.
Team ERA The ERA of all starting pitchers on the team averaged together.