In baseball, the batting average (AVG) is defined as the ratio of hits to at bats.
Henry Chadwick, an English statistician raised on cricket, was an influential figure in the early history of baseball. In the late 19th century he adapted the concept behind the cricket batting average to devise a similar statistic for baseball. Rather than simply copy cricket's formulation of runs scored divided by outs, he realised that hits divided by at bats would provide a better measure of individual batting ability. This is because of an intrinsic difference between the two sports; scoring runs in cricket is dependent almost only on one's own batting skill, whereas in baseball it is largely dependent on having other good hitters in your team. Chadwick noted that hits are independent of teammates' skills, so used this as the basis for the baseball batting average. His reason for using at bats rather than outs is less obvious, but it leads to the intuitive idea of the batting average being a percentage reflecting how often a batter gets on base, whereas hits divided by outs is not as simple to interpret in real terms.
In modern times, a season batting average higher than .300 is considered to be excellent, and an average higher than .400 a nearly unachievable goal. The last player to do so, with enough plate appearances to qualify for the batting championship, was Ted Williams of the Boston Red Sox, who hit .406 in 1941, though the best modern players either threaten to or actually do achieve it occasionally, if only for brief periods of time.
Ty Cobb holds the record for highest career batting average with .367, 9 points higher than Rogers Hornsby who has the second highest average in history at .358. The record for lowest career batting average for a player with more than 2,500 at-bats belongs to Bill Bergen, a catcher who played from 1901 to 1911 and recorded a .170 average in 3,028 career at-bats. The modern-era record for highest batting average for a season is held by Napoleon Lajoie, who hit .426 in 1901, the first year of play for the American League. The modern-era record for lowest batting average for a player that qualified for the batting title is held by Rob Deer, who hit .179 in 1991. The highest batting average for a rookie was .408 in 1908 by Shoeless Joe Jackson.
For non-pitchers, a batting average below .230 is often considered poor, and one below .200 is completely unacceptable. This latter level is known as "The Mendoza Line", named for Mario Mendoza, a stellar defensive shortstop who hit .215 during his Major League career. The league batting average in Major League Baseball for 2004 was just higher than .266, and the all-time league average is between .260 and .275.
Sabermetrics, the study of baseball statistics, considers batting average a weak measure of performance because it does not correlate as well as other measures to runs scored, and because it has little predictive value. Batting average does not take into account walks or power, whereas other statistics such as on-base percentage and slugging percentage have been specifically designed to measure such concepts. Adding these statistics together form a player's On-base plus slugging or "OPS". This is commonly seen as a much better, though not perfect, indicator of a player's overall batting ability as it is a measure of hitting for average, hitting for power and drawing bases on balls.
In 1887, Major League Baseball counted bases on balls as hits. The result of this was skyrocketed batting averages, including some near .500, and the experiment was abandoned the following season.