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 distribution together to characterize 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 or one skewed heavily toward fly balls, and that overall shape is often more diagnostic 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 consistently arrives with a slightly negative or flat angle skews heavily toward grounders; one whose barrel consistently arrives steep skews toward fly balls and pop-ups. A hitter with wide swing-to-swing variability in attack angle produces a scattered, inconsistent profile rather than a clear tendency in either direction, which is itself useful diagnostic information about mechanical repeatability.

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, while 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.