Memorial Day is traditionally the time of the year when the general public starts to take baseball seriously. But it also represents a milestone to statistical analysts like Baseball Prospectus, because two months of baseball is generally enough time for sample sizes to have reached a threshold of significance. Schedules have balanced themselves out, rosters have settled down, and the lingering effects of events like the World Baseball Classic have been muted.

With that in mind, let’s take a look at five hitters between the ages of 25 and 27 who have put together impressive – and unexpected – early-season performances. We choose these ages because breakout seasons are fairly common before 25, when many players are still growing into their bodies. A breakout season after the age of 27, on the other hand, almost always represents nothing more than a near-term fluke. Players between 25 and 27 are usually at the crossroads of their careers, and in that time define themselves as either a star or a scrub.

This analysis is assisted by Baseball Prospectus’s PECOTA projection system, which uses historical comparables to predict a player’s career prognosis going forward (Johnny Damon, for example, rates as highly comparable to Kenny Lofton and Andy Van Slyke). The stats that you see listed are a player’s projected batting average, on-base average, and slugging average (AVG/OBA/SLG), as well as his Breakout Percentage, which is PECOTA’s preseason estimate of the probability that a player will substantially improve his performance relative to his recent history.

ALEX RIOS, RF, TORONTO (25)
PECOTA: .278/.322/.433
17% CHANCE OF BREAKOUT
Actual: .351/.380/.649

Before the season began, there was room to debate whether Rios belonged in a starting lineup. This was a man who had hit just 11 home runs in about 1,000 big league plate appearances entering the season. With 10 home runs and 17 doubles thus far, Rios has not only rewarded the Blue Jays’ confidence in him, but entered the Memorial Day holiday as the league leader in slugging percentage.

That his breakout has been driven by power is particularly surprising. Even as a relatively well-regarded minor league prospect, Rios projected more as a jack-of-all-trades along the lines of Devon White than a legitimate power threat. The scouting reports show that Rios has made some productive changes to his game, lofting the ball far more than he had previously, and hitting to all fields. Nevertheless, his plate discipline remains poor, and he strikes out too frequently (once every six-at-bats) to maintain his .350 batting average. He should be a plus regular for some time to come, but don’t expect him to continue to flirt with superstar numbers.

NICK SWISHER, LF, OAKLAND (25)
PECOTA: .252/.347/.453
28% CHANCE OF BREAKOUT
Actual: .301/.398/.614

In Baseball Prospectus 2006,we urged Nick Swisher to be more aggressive early in the count, turning the tables on pitchers trying to take advantage of his reputation as a deep-count hitter. That is exactly what he has done: Swisher has cut down on his pitches seen per plate appearance, with seven of his 14 home runs coming on one of the first two pitches of the at-bat. Swisher is striking out even more frequently than Rios, though he maintains the high walk-rate habits of a power hitter of this typology. Expect Swisher’s batting average to settle back into the .260 range, but if he can be the American League’s answer to Adam Dunn, the A’s will have no complaints.

CHRIS SHELTON, 1B, DETROIT (25)
PECOTA: .280/.361/.493
14% CHANCE OF BREAKOUT
Actual: .322/.387/.603

Shelton’s breakout is not a complete surprise to statheads who have been touting his home run prowess ever since he was an unheralded catching prospect in the Pirates system. Certainly, much of the credit belongs to the Tigers and manager Jim Leyland, who punted incumbent Carlos Pena and gave Shelton the first base job. Our research shows that most major leaguers hit better when they’re in the everyday lineup, rather than being shuffled between several different slots in the lineup and the bench. Nevertheless, pitchers are already adjusting to Shelton in his second time around the league, and his high strikeout pace betrays some holes in his swing, particularly on pitches low in the zone. Of the players on this list, he has the least star potential going forward.

AUSTIN KEARNS, RF, CINCINNATI (26)
PECOTA: .275/.367/.510
32% CHANCE OF BREAKOUT
Actual: .297/.371/.535

Kearns’s 2006 should not be characterized as a breakout so much as a player getting back on track with what his fine early-career performance projected. In fact, as good as his numbers have been, Kearns has barely out-hit his (optimistic) PECOTA projection. Surely, keeping his health in order – and not being the subject of constant trade rumors – has also helped him to settle in at the plate. As with several other players on this list, Kearns strikes out too much to keep his batting average near the .300 plateau. Still, his fine all-around hitting approach could make him the Dwight Evans to Adam Dunn’s Jim Rice.

NICK JOHNSON, 1B, WASHINGTON (27)
PECOTA: .268/.388/.443
12% CHANCE OF BREAKOUT
Actual: .302/.417/.566

Nick the Stick’s track record suggests that the longer he stays in the lineup, the better his performance turns out to be. Johnson has now gone 10 months since his last major injury – amazingly, this represents his healthiest streak since winning the Yankees’ first base job as a 24-year-old in 2002. He is capable of maintaining this performance if healthy, and players with as intelligent a hitting approach as Johnson are no strangers to mid-career power breakouts. Still, the inertial rule of injuries suggests that a player who starts getting injured tends to keep getting injured, and Johnson and his brittle wrists have broken our hearts one too many times for us to buy in completely.

Each of these players has emerged as a significant long-term asset to his team, and all but Johnson figure to play a prominent role in this year’s pennant races. But ranked from most to least likely to maintain their current pace, we would order them: Kearns, Swisher, Johnson, Rios, and Shelton.

Mr. Silver is a writer for Baseball Prospectus. For more state-of-the-art commentary and analysis, please visit www.baseballprospectus.com.