Ground-ball rate represents the percentage of balls hit into the field of play that are characterized as ground balls. Each ball that is hit into the field of play is characterized as a line drive, a fly ball, a ground ball or a pop-up.
Ground-ball rate can be used as a metric to evaluate both hitters and pitchers, although it's more frequently used to evaluate pitchers.
With pitchers, ground-ball rate can be very telling. For one thing, it lets us know what type of pitcher we have. Pitchers with high ground-ball rates have a tendency to allow fewer home runs (which result from fly balls and line drives). Obviously, preventing home runs is one of the most important aspects of pitching -- as no outcome is more damaging.
Pitchers who possess high ground-ball rates have a tendency to induce a high number of double plays as well, and are generally more successful than pitchers who try to get by with high fly-ball rates.
Watch: Jake Diekman induces a ground ball, increasing his ground-ball rate.
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