Okay, I'd posted this in the comments section of another post a few days after it had been originally posted, but immediately realized that no one will read it there, so I'm putting it up here again.
A lot of people like to look at a player's BABIP, and if it's above league average, they say that player has been lucky--if it's below league average, they say that player has been unlucky. Others refine their statements somewhat by comparing that player's BABIP to an expected BABIP that is generated predominately from the player's line drive percentage. I am wholly unconvinced of the legitimacy of either method and have become increasingly frustrated with these seemingly unfounded proclamations. Do you feel likewise? Read on!
My premise: Ignoring wind and sun-aided hits, whether a ball in play is recorded as a hit is a matter of it's velocity and its trajectory, wherein a ball's trajectory is essentially it's vertical angle relative to the ground, and its lateral angle relative to the left and right field lines.
I'm willing to grant that the latter is predominately a matter of luck--a low line-drive straight up the middle is a single, but if it's 10 degrees to the right or left, it will caught by a middle-infielder. xBABIP, you ignore this matter entirely and for good reason. You're quite shaky on the rest though.
A ball's vertical trajectory determines whether it is called a ground ball, a line drive, or a fly ball, and a batter's ability to affect his batted ball's vertical trajectory over the course of a season is absolutely NOT a matter of luck. Alex Gordon generates twice as many fly balls as Joey Gathright. If they're both still playing in 5 years, Alex Gordon will still be generating twice as many fly balls as Joey Gathright. I know most xBABIP calculations take things like LD%, GB%, and FB% into account, but these 3-way classifications are not sufficiently specific, nor are they especially accurate. One fly ball is not the same as the next, and it's not even always clear whether a batted ball is a fly or a line drive. Likewise, a one-hopper through the infield has a lot more in common with a line drive than it does a ground ball pounded straight down into the dirt.
xBABIP calculations that rely on LD/FB/GB percentages are assuming that the majority of one player's flies, grounders, and line drives are leaving the bat at roughly the same vertical angles as the next. This assumption is certainly flawed, but at least the calculations are taking the vertical angles of batted balls into account when determining their probability of generating hits. The much more serious flaw in these calculations is that they don't even take the velocity of batted balls into account. Certain players consistently hit balls harder than others. Likewise, certain players consistently hit balls more weakly than others. And the velocity of a ball of the bat has a HUGE impact on whether it generates a hit. A weakly hit fly ball is called a pop-up and is a nearly automatic out; a hard hit fly ball lands behind an outfielder for a double. Even a ground ball that's hit straight down will generate a hit if it's hit hard enough to produce a large bounce. Likewise, a line drive is an easy out for an infielder if it's hit softly.
Any calculation that purports to generate a player's expected batting average on balls in play without taking into account the speed of that player's batted balls is neglecting one of the most important factors in that expectancy. Because of this, we end up calling players who consistently make hard contact lucky, and players like TPJ who consistently fail to get the barrel of the bat on the ball extremely unlucky. Does this make sense?