Over at FiveThirtyEight, Neil Paine has an interesting post on what sort of expectations we should set for DeSean Jackson. This is a good question, but it raises a second, wider question: What should we expect DeSean Jackson to mean, on net, for his new offense as a whole?
As Paine identified, since 1970, there have been 24 wide receivers—including Jackson—who posted an "Approximate Value"* of at least 10, and then changed teams over the offseason. 20 of those receivers saw a drop in approximate value in their first season with a new team, with Jackson TBD.
Paine acknowledges that Jackson is younger than most of the receivers he identified, and that any list of "top performers" is likely to regress. But even if top receivers drop off, they can still attract defensive attention and free up their teammates (in fact, this additional attention may be why they dropped off). Even if Jackson's approximate value drops in 2014, should we expect Washington's offense to get better?
Going back through Paine's list, two receivers (Willie Jackson and Brett Perriman) played for multiple teams in the year after their move, so we won't include them here. If we're looking at team offensive performance from one year to the next, we have to throw out nine teams that saw a significant QB shift the same year they added a star wideout; teams that had the same QB start the majority of games in both years counted as "stable".
That leaves us with the twelve receivers below, sans Jackson. The three players Paine identified who didn't drop off are on the list, but we still have nine guys whose approximate value took a big hit when they changed teams. For each of these squads, we looked up the passing yards per game and yards per play (passing and rushing combined) for the season before and after the receiver was added. The AV stats that Paine uses are also included.
|Team||New WR||Year||WR AV Before||WR AV After||Pass Y/G Before||Pass Y/G After||Tot. Y/P Before||Tot. Y/P After||Δ Y/G||Δ Y/P|
10 of the 12 teams saw an increase in passing yards per game with the new receiver, for an average increase of 15.2% overall. This just tells us that teams were getting more pass-focused, which is self-fulfilling if a team just invested in a new wideout. So more telling is that 8 of 12 teams saw an increase in yards per play, for a 3.4% average increase overall.
We're dealing with a small sample here, and while the difference in passing yards per game is statistically significant (two-tailed/paired, p= 0.002), the difference in yards per play is not (p=0.11). Great receivers don't change teams often enough for us to say anything confidently, but the numbers let us know to be careful: Jackson's stat line in his first D.C. season may not be telling the whole story.
*Here's (roughly) how "Approximate Value" works: Calculate the team's offensive value relative to the league, divvy up that value to the offensive line/rushers/passers/receivers depending on usage and estimated importance of each group, and then divvy up those values within the groups based on relative usage (for receivers, calculated as [receiving yards]/[team receiving yards]).
I don't love it as a stat; the calculations involve some constants comparing positional importance that are based on drafts and haven't been updated since 2007/2008, and the offensive line stats—which in turn determine how much "value" is left for skill players—incorporate stats on all-pro and pro-bowl selections, which aren't incorporated into any other positions. Lots of very different pieces of data go into AV, and it's pretty messy.
Photo via @Redskins