How do you tell the difference between a flash in the pan and the start of a trend? Sometimes changes happen so gradually you can't even notice it until after the change actually occurs, like the rising strikeout rate over the last decade:

There is no one season where you could say, at the time, "Aha, this is where the change occurred." And yet, over the past decade, strikeout rates have jumped up nearly 25 percent, a substantial leap. In retrospect, the trend is clear -- in the summer of 2006, it wasn't quite.

On the other hand, you have something like the 2014 rookie wide receiver class in the NFL. The 2014 class saw five different players finish with at least 800 yards and five touchdowns -- the most in a single season, and nearly 10 percent of the total history of the league. With the direction the league is moving, toward more and more passing, this was surely the start of a new trend of young receivers stepping into the league and making a big impact from day one, right?

Not so much. The 2015 rookie class had just one 800-yard, five-touchdown performance, just the second time in the last five years there has been just one. 2014 may yet end up being the start of a trend, but if you bought into that last season, you probably wasted a draft pick on the likes of Kevin White, DeVante Parker, Nelson Agholor, Breshad Perriman or Phillip Dorsett -- so sorry.

We're going to have a similar question to answer in baseball, after last year's similarly historic rookie class. We had 14 rookies reach 300 plate appearances last season with at least a league-average OPS, one of the highest marks in league history -- only 2006 topped it. And if you increase the level a bit, the class stands out even more; the seven players who finished with at least a 120 OPS+ are the most since at least 1936.

You really don't need to be introduced to last year's rookie class, but anyone who won a Fantasy championship last season knows how important and valuable names like Carlos Correa, Kris Bryant, Miguel Sano and others were. And if you missed out on those stars -- or others like Randal Grichuk, Francisco Lindor, Jung Ho Kang or Maikel Franco -- you know how costly your miss truly was.

But don't over-correct. Last year might have been the start of a trend of younger players hitting the ground running in the majors -- there is some evidence that players are peaking earlier than ever -- but there's a better chance 2015 was a historical outlier. Only three players each in 2014, 2013 and 2012 reached the 300 PA, 120 OPS+ mark, and the fact that only one rookie pitcher managed to post a 120 ERA+ in 100-plus innings (Lance McCullers) would seem to suggest there isn't much evidence to support the idea that last year was the start of some trend toward younger stars.

This isn't an argument against any specific player, and it seems like Fantasy players are being pretty smart about keeping expectations in line for the rookie class; only Corey Seager is really being drafted near the top of most drafts, and his No. 63 overall ADP via is hardly unreasonable given how well he played last season.

Corey Seager
LAD • SS • 5
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However, it would be easy to be overrate even Seager, who took the league by storm in 27 games last season with a .986 OPS. He won't be able to sustain that kind of production for a full season, and as a young lefty he'll have to overcome some of the same issues that plagued fellow Dodgers youngster Joc Pederson a year ago. Pederson was tremendous early in the season, but his platoon issues and possibly fatigue caught up with him in the second half and severely limited his appeal as the season went on.

Producing in the majors from day one is tough. Even some of the most talented players to ever play the game struggle. The pressure is higher, the competition is tougher, the scouting is better and the season is longer, all of which can add up to a tough road for young players. Last year's crop of young players was among the most talented to collectively make the jump at once, so their collective success will be tough to replicate any year.

As you get set for the season, just continue to remind yourself this: Most rookies struggle. That last year's crop managed to be so impressive doesn't change that simple fact, and if you go into this season expecting a handful of rookies to carry you to glory again, history tells us you're likely to be sorely disappointed. You can bet on the outlier becoming a trend, but it's not a smart one.