Baseball Prospectus has a metric called Wins Above Replacement Player, which (ideally) should be a pretty straightforward proposition. But let’s look at a few things. I have a collection of sortable WARP1 reports from October 9th, covering 1996-2009. How does it match up what’s currently on the site?
Currently, BPro says Vlad was worth 59.1 WARP1 in that timespan. Based on what was reported on the 9th, Vlad was worth 69.8 WARP1.
Okay, so that’s just one player. Let’s try… Scott Rolen. 75.9 WARP1. A few weeks ago? 82.4.
Nomah? 49.6 WARP1, compared to 59.0 WARP1 before.
So what’s going on here? Aggregate WARP1 per season has declined by roughly 150 over the past few weeks. That’s right – nearly 150 wins per season have simply disappeared from Baseball Prospectus’ website.
Why? It seems that BPro has changed how they convert runs to wins. Before, they were using a scale where 8.2 runs = roughly 1 win. Now it’s a lot closer to 9.2 runs per win. (For a fuller explanation, check out this discussion of the subject.)
This is, by all accounts, good (it has the nice side effect of making individual player WARP sum up to team WARP, assuming a replacement level of .220 – we can have that conversation another time). Just one question – why hasn’t BPro announced this change?