This article is part of our MLB Barometer series.
I'm not sure I've ever met a stat with an "X" in it that I didn't like. There's a beautiful simplicity in how a good X stat mirrors a results-based stat we're all familiar with while giving us greater insight about a player's underlying process. Take xBA (expected batting average), for example, which can tell us things like, "Kyle Tucker may be a .278 hitter this year, but he's hitting the ball like a .308 hitter."
Such stats do of course have their limitations. You might see frequent warnings about how X stats aren't intended to be predictive, though to be fair to them, results-based stats like raw batting average aren't predictive either. Tucker's .308 xBA just means that his contact has been typical of a .308 hitter, not that he'll hit .308 next month or next year. Likewise, though, his .278 average just means that he's reached base via a hit in 27.8 percent of his at-bats so far this year, not that he'll continue to do the same going forward.
The main limitations for X stats is that they're simply not built to include everything. xBA and xSLG, for example, look at launch angle and exit velocity, but don't currently include spray angle. xFIP, my favorite X stat on the pitcher side, measures a pitcher's performance by his strikeout, walk and groundball rates, but doesn't include any other information about batted-ball quality beyond whether it was hit on the mound.
Despite those flaws, I find myself turning