The thing about sample size, though, is the standards for a reliable one differ by statistic. For, say, a pitcher's strikeout and walk rates, we only need a few hundred batters faced, which typically takes a month or two. On the flipside, defensive metrics require much more time to stabilize — about 3,000 innings in the field, generally acquired over three-plus seasons. For someone like Adam Jones, whose play with the glove has spiked going back to last year, this means that we can't fully believe in what he's done yet; nevertheless, there's a lot here to suggest that he'll keep it up.
Let's go back to Jones in the minors. Drafted by Seattle in 2003, he came up as a shortstop, only switching to the outfield in 2006. He adjusted pretty well to the position, or so scouts observed; the 2008 BP Annual called him "one of the best defensive outfielders in the minors". His first season on the East Coast saw him live up to that reputation, as he gained 6 DRS and 5.9 UZR for the Orioles, ranking a respective fourth and sixth in the majors. Only 22 at the time, Jones looked like an elite defender for years to come.
That didn't exactly come to pass, it seemed. Over the next five years, Jones cost the Orioles an unacceptable number of runs in the field — 28 by DRS (second-worst in baseball), and 33 by UZR (third-worst in baseball). The takeaway from that*: Jones didn't make his keep on the field. Such a sizable sample as that didn't leave much room for doubt, nor did it endorse Jones's abilities going forward.
*Aside from noting that, good Lord, Matt Kemp cannot play defense.
After that, 2014 came, then 2015. (That whole passage-of-time thing — it's crazy how that works.) In these years, Jones has turned over a new leaf; he's earned 13.5 runs by UZR, which places him fifth in baseball, and 9 runs by DRS, which places him sixth in baseball. After a long period in the relative cellar, he has, possibly, come back to reclaim his throne.
As discussed above, defensive metrics take some time to stabilize. These results should therefore inspire some skepticism, and further scrutiny. Let's look at his Inside Edge fielding numbers — they only go back to 2012, but his output from 2012 to 2013 (-18 DRS, -13.4 UZR) effectively represents his play over the aforementioned five-year span.
Inside Edge breaks down fielding into six types of plays: 0% (the impossible plays), 1-10% (the remote plays), 10-40% (the unlikely plays), 40-60% (the even plays), 60-90% (the likely plays), and 90-100% (the routine plays). Each percentage range signifies the theoretical chance that a defender can successfully field the ball. Because scouts gauge these manually, they're subjective, but they tend to properly evaluate most defenders. And for Jones, they show a clear change:
While the tougher plays haven't shifted much for Jones, their easier counterparts have. This syncs up with the narrative surrounding him as well: Quite notoriously, Jones used to botch more than his fair share of simple plays in a season. For me, this error against the Yankees sticks out, on account of its most prominent feature:
Yes, Jones blew the controversial bubble on this routine (per IE) play, and paid a heavy price for it. In his darker times, Jones would treat Oriole fans to a few of these a year, the gum only compounding the subsequent frustration. Since then, however, he's managed to avoid this ignominy, even making a superb play while blowing a bubble.
This kind of defensive improvement is likely the most sustainable. Had Jones simply reeled in a few highlight catches, they may have pumped up his metrics, but they wouldn't testify to any true change in his performance — despite the low probability of converting one, every player lucks into a few sooner or later. By contrast, the capacity to convert easy balls (the most common variety) into outs will build up a player's numbers gradually, in a hopefully non-fluky manner. Plus, fielders who derive their value from easy plays will presumably age better; as their athleticism and speed deteriorates, they can still rely on solid fundamentals to give them a lift.
Perhaps this step forward will vanish in the final months of the season, a statistical mirage among the desert of subpar defense. Jones's track record certainly contradicts what he's accomplished over the last year-and-a-half, and in this area, the past often predicts the future. But this improvement has some legitimacy to it — the scouts, always more trusted with these cases, back up the numbers. If Jones does sustain his better play in the field, he might live up to the high standards he set for himself way back in the minors. While fielding metrics are fickle, here they appear to have stumbled onto an actual trend.