On Fan Bias

You may have noticed that Fangraphs is all set up for community projections now. This is the same thing USSM/we used to do, only for everybody in baseball. It's a really cool feature that's remarkably simple to use, and one neat little twist they threw in was including the rooting interest of the user inputting data. You might remember that, when we would run these things every year, there would always be some level of concern that, being Mariner fans, we might be a little optimistic about our own players. Fangraphs now allows us to investigate that possible bias.

There are five Mariner hitters who, so far, have received at least ten projections from both Mariner fans and non-Mariner fans. Here are their projected 2010 wOBAs:

Player M's fan non-M's fan
Ichiro 0.359 0.359
Guti 0.344 0.329
Lopez 0.327 0.325
Branyan 0.369 0.349
Beltre 0.349 0.346

No difference for three of them. Guti gets a little bit of a 2B/HR boost, while Branyan's difference comes down to 30 homers in 508 PAs versus 24 in 475. It could be bias, or it could be that M's fans know a little more about their players than people who didn't watch them as much.

(Both Griffey and Jack Wilson have a bunch of projections from Mariner fans and 5 from non-Mariner fans. There is no meaningful difference. Felix Hernandez, meanwhile, has FIP projections of 3.05 and 3.30 over equivalent innings.)

Looking through these numbers, and going through some of the early projections for players on other teams, two things become apparent:

(1) In nearly every circumstance, the fan projection is equal to or better than the non-fan projection

(2) Overall, the difference between the fan projections and the non-fan projections is small

Does optimistic fan bias exist within this exercise? It does appear to, yes. On average, fans are a bit higher on their own players than they are on the rest. But the effect of this bias is so small that, for all intents and purposes, it doesn't seem to matter. I suppose that, if you really wanted to, you could come up with a list of reasons to be skeptical of the Wisdom Of Crowds approach to player projection, but you can probably go ahead and cross "bias" right off.

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