Yesterday Ivan Rodriguez was traded from the Tigers to the Yankees. This made sense from one perspective since the Yankees were in desperate need of a catcher and Pudge was available. But this trade raises several questions:
1. Is Pudge really a good hitter?
2. What about his hitting ability at this point (age 36) in his career?
First, Pudge is noted for his high career batting average of .302. But this is a bit misleading, since AVG ignores other ways for a batter to get on base such as a walk. From a OBP perspective, Pudge has a mediocre batting record and also he hasn't displayed much power in recent years. Pudge will be an ok hitter towards the bottom of the Yankees lineup since he is good in hitting singles. But Pudge is a good example of a hitter who looks better than he is based on his batting average.
To understand how Pudge's performance over time, we plotted his batting averages as a function of age. We see that his AVG showed steady improvement in early years, hit a peak in midseason and has displayed a lot of variability (up and down movement) in recent years. How do we make sense of these large changes in AVG in the latter part of Pudge's career? This illustrates the difficulty in learning about a batting ABILITY based on a batter's PERFORMANCE in a single season.
We talked about ways of learning about a player's ability -- that is, his hitting probability -- based on x successes in n at-bats. This discussion showed how little we learn about ability based on a single season. We get a better understanding of a player's ability by looking at his performance over several seasons.