Should We Have Seen Jeter’s Drastic Decline Coming?

Derek Jeter is clearly in decline. With each passing at-bat he further cements that fact, as he shows little sign of recovering any sort of pop and looks slow in all facets of the game. While it is possible that he has some sort of recovery and raises his stat line a bit, we have likely seen the last of the Derek Jeter who was consistently one of the best shortstops in the game. This decline has come as a surprise to many, as Jeter’s 2009 seemed to suggest that he had a number of productive years left, and his sudden slide towards replacement level has left us scratching our heads. While I think most of us expected some sort of steady deterioration due to age, this rapid descent into mediocrity has seemingly come from nowhere. The question is, should we have seen this coming?

From a purely statistical point of view, I do not think that the decline was predictable. Let’s look at the data, courtesy of Fangraphs:

While some have pointed out that removing 2009 from Jeter’s ledger makes this look like a steady decline, I think that is misleading. If you look closely, you will see that 2006 was Jeter’s best year at the plate since 1999. The career year creates the illusion of a subsequent decline that is not entirely accurate. Here is what I wrote on this issue prior to the 2009 season:

There is one important point that needs to be made here. 2006 was one of Derek Jeter’s best seasons, and was out of line with the seasons surrounding it. The data Mike posts is therefore a bit misleading. Jeter’s 2007 numbers are actually right in line with 2001-2005. When looked at in this broader context, he has had one season of decline.

Essentially, 2007 was not a decline. Rather, it was simply a return to the baseline (.370 wOBA/120-130 wRC+) that Jeter had settled into starting in 2002. The first real negative deviation that we saw came in 2008, but the return to the baseline that came in 2009 rendered 2008 a statistical outlier. Looking at the data following the 2009 season, it was reasonable to assume that 2008 was just an off year and that Derek’s impending age related decline would have a statistical starting point somewhere around his baseline numbers. The CAIRO projections for 2010, which pegged Jeter for a .353 wOBA, show that the projection system felt the same way, as that number represents a reasonable decline from the 2001-2007 baseline.

Of course, we now know that 2009 was likely the blip on the radar and that Jeter began a decline in 2008 that continues to get drastically worse. Instead of a gradual decline starting from the 2002-2007 baseline, Jeter fell off drastically in a manner that seemed to spring from his 2008 numbers. So what did we miss? I think friend of the blog @JoeRo23, who started this discussion yesterday on Twitter, was on point when he said that age was not considered enough when evaluating Jeter’s future. For a younger player, an outlier can be dismissed as an off year and might only have minimal impact on our projections. However, when it comes to a 34-year old shortstop, a season like 2008 should probably be given greater weight than it actually received. A strong 2009 obscured the fact that an aging player had recently put up the worst season of his career, and that it might make sense to consider that off year heavily when trying to determine a baseline from which to estimate his decline.

Let’s return to the question posed in the title: should we have seen Jeter’s drastic decline coming? I think the answer is that while we could not have expected a decline this poor based upon the statistical record I examined above, we probably should have expected a greater level of deterioration than we did. The pure data suggested that the decline had not yet begun, but Jeter’s age should have told us that using the baseline established from 2002-2007 was a mistake, no matter what happened in 2009.

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