The biggest point of controversy with DRS lies in its sample size. Since DRS uses only one year of data to compare fielders, it's not pulling from a large set of data. This is done to compensate for possible year-to-year changes in around the league. DRS operates under the belief that teams will utilize better defensive players as the league continues to recognize the importance of defense, and that those numbers should not be compared to past seasons, when the league didn't value defense as much. There's a reason for the adjustment, but it also leaves DRS with a smaller sample of data to pull from.
Another way writers often use these numbers is to say that once a hitter reaches a threshold in a particular year, it becomes safe to assume that he has reached a new level of production. But that’s also not true. Just because a hitter reaches, say, a 100 PA threshold doesn’t mean that every plate appearance before the most recent 100 are meaningless. Recent performance is more important than older performance, but older performance still matters. What we should do is weight the older performance less, include it with the recent performance, and then use our threshold to determine how much mean performance we need. So while a player may have just reached the 100 PA threshold, he may have 300 effective plate appearances once we account for past data, in which case we’d use 75 percent of the player and 25 percent of the mean.
These data, called statistics, can show patterns that otherwise would be hard to see. Baseball is full of statistics, such as data on which players are hitting better than they used to, and which aren’t. In a December 2012 paper published in the research journal PLOS ONE , researchers found that players perform better when they’re on a team with a slugger who is on a hitting streak. Other researchers may compare statistics from different years to look for longer-term patterns, such as whether baseball players overall are getting better or worse at hitting.