Exit Velocity measures the speed of the baseball as it comes off the bat, immediately after a batter makes contact. This is tracked for all Batted Ball Events -- outs, hits and errors.
Attaining a high Exit Velocity is one of a hitter's primary goals. A hard-hit ball won't always have a positive result, but the defense has less time to react, so the batter's chances of reaching base are higher.
For this very reason, Exit Velocity can also be used to evaluate pitchers (known as "Exit Velocity Against"). The game's best pitchers -- who naturally throw the highest quality pitches -- generally rank among the league leaders at limiting hard contact.
Average Exit Velocity (aEV) is calculated by dividing the sum of all Exit Velocities by all Batted Ball Events.
Hitting the ball hard (or limiting hard contact, in the case of pitchers) is skill-based and therefore considered at least somewhat predictive of future performance. If a player is struggling statistically but registering impressive Exit Velocity readings, he may be a candidate to bounce back in the near future. For example: Say Player A hit .200 in April but ranked among the league leaders in average Exit Velocity; a wise fantasy owner could look at Player A's average Exit Velocity and surmise that he will eventually regress toward his career statistical averages. The same exercise can be done with players who are exceeding expectations; if a hitter's Exit Velocity is low, he is less likely to maintain his torrid pace at the plate.
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