May 8, 2013
It's my pleasure to inform you that we have started producing PECOTA updates for rest-of-season performance.
Before we get into the details, let's clarify what is being updated. The changes will affect:
For players who are in the depth charts, the PECOTA summaries at the top of the player cards will be updated as well. Players who are not in the Depth Charts but have preseason PECOTA projections will still have those at the top of the cards. Any other PECOTA data on the cards, including PECOTA percentiles, will be unaffected. In order to view the Depth Charts or use the Player Forecast Manager, you must have a monthly, fantasy, or premium subscription to Baseball Prospectus.
There are three principal changes to the way we're calculating in-season PECOTAs this year. One is how playing time is handled. We are still using the Depth Charts to estimate a player's playing time, but we are now using games remaining on the MLB schedule to determine how much playing time to allocate to each team (and from there to each player).
The next is the projections themselves. PECOTA works by forecasting a series of component stats (some easily recognizable, like walk rate and power on contact, some more obscure) and then building batting lines and eventually value stats. We assign each of those components a reliability score, based on our estimate of the margin of error of those component projections. (The major inputs are the amount of playing time the player has had, how long ago that playing time was, and the level—MLB, Triple-A, etc.—the playing time was at and the spread of talent in that population.) For season-to-date stats, we compute the same components and the same reliability scores. We then combine the two, using the reliability as the weight—in essence, we treat the preseason PECOTA as a Bayesian "prior." (You'll note I didn't mention regression to the mean—since the preseason forecast is already regressed, we don't need to regress the current season stats any—in essence, we're "regressing" the player's stats to his preseason projection, not the population mean.)
The third is that we are updating projections daily, rather than whenever the depth charts change. That way, changes in playing time due to the remaining games diminishing are captured, as well as changes to the PECOTAs based on player performance (although a single game's performance is highly unlikely to move a player's forecast in a noticeable way).
Again, as a reminder—only the PFM and DCs (and products that rely on them) and the PECOTA summaries at the top of the player cards of players in the DCs are affected. Every other PECOTA product is still using the preseason forecast. If you have any other questions, please leave a comment below.