Thanks for all of the e-mails with questions and suggestions for the SSPS. The fact that the system projected a reasonable bounceback for Chipper is very interesting. So, I went and took a closer look.
Chipper was really unlucky on balls in play last year. The effect of the batting average on balls in play on a player’s overall batting average is huge. It explains close to two-thirds of the variance in batting average; however, it explains next to nothing about the players batting average the following year. For example, if Chipper had batted .300 on balls in play last year (instead of .246) his projected batting average would rise by a only 5 points. Chipper’s walk-rate and homer-rate, and extra-base-hit-rate were 1.75, 1.6, and 1 standard deviations above the 2004 average for players in the sample. His K-rate was average. His BABIP was 1.4 SDs below the sample average (also well below his career average of .311) but it just didn’t matter in the projection. This explains why his batting average was so bad last year but projects to be good this year. All of his other stats are on par with his career averages. His homers and extra-base hits were up a bit, but not by much. It’s just that guys who strikeout, walk, and homer like Chipper did last year, usually have a much higher batting averages.
One very odd prediction that a reader noticed is that Ichiro, a career .340 hitter coming off a career high .372 AVG, projects to hit .292. So, let’s look at the his projected (actual) batting averages for the past 3 years: .292 (.321), .292 (.312), and .285 (.372). The SPSS is not doing so hot, but that’s not surprising given the uniqueness of Ichiro. His speed had to be an asset in terms of reaching base on balls in play at a higher rate than most players, which my system does not reward. This is because the correlation from year-to-year is tiny so it is not assigned much weight in the regression. Ichiro averaged a BABIP of about .360 (with a whopping .400 in 2004) compared to a league average of about .300. So, this is one of the things I need to work on.