Spring training numbers are generally unimportant for a few reasons. First, they are based on small sample sizes. We've all seen weaker hitters get off to strong starts for the first three weeks of the regular season, only to crash to earth by the middle of May. Players can go hot and cold in spring training as well.
Spring performance is especially tricky because the competition is weaker. There are a lot of players used in spring training games, such as career minor leaguers and rookies not yet ready for the majors, who will not be playing once the season starts. It's a lot easier to pile up hits against minor league pitchers than against major lerague pitchers every night.
Finally, players are not always playing to win in spring training. A pitcher may be working on a new pitch or delivery. A batter might be trying a new stance or a different approach. Veteran players, already guaranteed a roster spot, might simply be going through the motions just trying to stay in shape while avoiding injury.
So, it's easy to see why most spring numbers are generally meaningless. However, John Dewan, owner of Baseball Info Solutions, did a study on spring statistics a few years ago and concluded that one statistic does have some meaning:
A hitter with a positive difference between his spring training slugging percentage and his lifetime slugging percentage of .200 or more correlates to a better than normal season.
So, if a player has a .400 career slugging average during the regular season and he is slugging .650 during spring training, you might expect him to slug significantly higher than his usual .400. in the coming season. There is no correlation for other statistics such as batting average, on-base average and ERA.
Dewan says that at least 35 spring training at bats are required for his theory to have meaning. Nobody has 35 at bats yet but if, in a couple of weeks, Magglio Ordonez is still slugging .929 and Don Kelly is at .882, it might be worth noting. We'll visit this again later.