Pop-up rate represents the percentage of balls hit into the field of play that are characterized as pop-ups. 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.)
Pop-up rate can be used as a metric to evaluate both hitters and pitchers, although it's more frequently used to evaluate pitchers.
Pitchers with high pop-up rates are generally successful because fly balls to the infield almost always result in outs. However, often times a high pop-up rate correlates to a high fly-ball rate, which in turn correlates to a high home-run rate.
Pop-up rates are used far less frequently as an evaluator than line drive, fly-ball and ground-ball rates. In fact, the primary purpose of recording pop-ups is to distinguish them from fly balls, which can be far more harmful to a pitcher.
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