2009 Is Not A Constant by Dave Cameron December 28, 2009 I would like to make a plea with anyone who plans on writing anything about how a team’s off-season has gone – please, please, please do not treat 2009 as a constant. This happens all the time. Team X added Player Y but lost Player Z, so they are about the same as they were. Or sometimes it will appear in a slightly different form – the team brought in this guy to replace the big hole they had last year, so with that improvement and no obvious downgrades, they’re going to be even better! All of these statements presuppose that each team should expect to get something close to equal to the 2009 performance of all the players they are retaining from last year’s roster. But that’s not even remotely close to true, and everyone knows it. We all know about career years and how you have to expect regression after a player does something way outside the ordinary, but regression doesn’t just serve to bring players back to earth after a big year. Regression “fixes” a lot of problem spots from the prior year, even if the team doesn’t make a serious effort to change out players. The Royals got a .253 wOBA out of their shortstops a year ago. I don’t care how bad you think Yuniesky Betancourt is, you have to expect that number to be higher this year. They didn’t do anything to improve their shortstop position this winter, but the level of production they got from the position in 2009 is not their expected level of production for 2010. It isn’t just individual players performances, either. Last year, the Phillies had a LOB% of 75.0% despite a pretty mediocre 4.36 FIP as a team. They stranded a ton of base runners, something that is not an easily repeatable skill. They could pitch much better, thanks to the addition of Roy Halladay, and still give up more runs than they did a year ago. This applies across the board. Injuries, clutch hitting, variance in run distribution – all of these are subject to extreme amounts of regression, and they all had a significant impact on how some teams performed last year, both in terms of “raw” wins and losses and things like runs scored and runs allowed. You cannot just look at a team’s prior year won loss record – or even their pythagorean record – make some adjustments for the off-season transactions, and presume that’s a good enough estimator of true talent for the 2010 team. However, it’s done all the time. I’m not just talking about mainstream writers here – you’ll see this kind of “analysis” on pretty much every blog and commentary about baseball, sabermetric or otherwise. For whatever reason, it’s become acceptable to just plug in the new guys numbers in place of the old guys numbers and call it a day, pretending like everything else from last year is not going to change at all. That’s lazy and it’s wrong. Don’t do it.