Signature Significance

If there is a theme to my April baseball watching and writing it is this- beware small sample sizes. We have seen time and again players who get out of the gate quickly only to fall apart after the calendar turns to May. Joel Sherman provided  a good example the other day with Vernon Wells. Another good example from that 2013 Yankee team is Travis Hafner. Hafner had 6 HR’s and a line of .318/.438/.667 when April ended and he finished with 12 HR’s and a line of .202/.301/.378 for the season. Those two examples are why we should be very, very, careful to avoid making any conclusions about the rest of the season from the results so far, and why the following should be looked at with a skeptical eye.

In an Economist blog post the other day the author took a look at Alex Rodriguez’s 477-foot home run on Friday and what it means for the rest of the season. I encourage you to read the article, but the key takeaway is the concept of signature significance- an idea named by Bill James that says that certain rare results have much higher predictive power than one game or even one swing normally should. Hitting a 477-foot home run is that type of result. The key quote:

The fact that Mr Rodríguez propelled a single baseball 477 feet means there is a very strong chance he is not the player we thought he was. Guys who are washed up just don’t hit 477-foot homers. Not even once.

I’m still wrestling with this idea and its predictive powers, but I find it a fascinating concept.