Fly-ball rate represents the percentage of balls hit into the field of play that are characterized as fly 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. (A fly ball is a fly to the outfield, while a pop-up is hit to the infield.)
Fly-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, fly-ball rate can be very telling. For one thing, it tells us what type of pitcher we have. Pitchers with high fly-ball rates have a tendency to allow home runs (which result exclusively from fly balls and line drives). In this regard, pitchers generally strive to avoid high fly-ball rates. But some pitchers can thrive with a high fly-ball rate as long as they limit their home runs, because outs occur more frequently on fly balls than on ground balls.
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