Ground Ball vs Fly Ball Tendency

Ground ball rate and line drive rate each describe one slice of a hitter's outcomes. Tendency looks at the full spread together to describe a hitter's overall swing-plane bias. A hitter whose grounders vastly outnumber their fly balls is telling a different mechanical story than one with a roughly even split. The same is true of one skewed heavily toward fly balls. That overall shape often tells you more than any single rate viewed alone. Two hitters can share an identical line-drive rate while having very different ground-ball-to-fly-ball ratios on their remaining batted balls.

Tendency is driven primarily by average attack angle and its consistency. A hitter whose barrel reliably arrives with a slightly negative or flat angle skews heavily toward grounders. One whose barrel reliably arrives steep skews toward fly balls and pop-ups. A hitter whose attack angle varies widely from swing to swing produces a scattered, uneven profile rather than a clear tendency in either direction. That itself is a useful clue about how repeatable the mechanics are.

Tendency isn't inherently good or bad in either direction. A contact-oriented hitter using the whole field may deliberately favor a flatter, grounder-friendly profile to beat a shift. A power hitter's profile skewing toward fly balls is consistent with (though not proof of) a productive, on-plane attack angle. Reading tendency alongside hard-hit rate and line drive rate together gives a much fuller picture than any one number alone.