Park factor fix for Forecasts

The Forecasts update posted yesterday, Feb. 18, will have different numbers for every player from the week before, as I discovered a logic error in my code which was preventing park factors from being applied to each player’s batting and pitching projections. Recently published Forecasts were park neutral but now are as intended, specific to the player’s team.

Major league players’ projections are customized to the weighted mean of all the ballparks his team played in the previous season. Projections for players in the minor leagues are based on their parent major league team. Schedules for 2012 may now be downloaded from mlb.com, and as soon as those are imported into the Oliver database, I will use the number of games scheduled to be played in each ballpark in the coming season instead of the previous.

I do not expect using 2012 instead of 2011 to produce major changes in the projections, but some players who play in extreme parks did see sizable differences when their park factors were correctly applied. Troy Tulowitzki solidified his rank as the overall best position player; playing half his games in Coors Field inflates his batting projection from a park neutral .285/.358/.506 to .302/.371/.544. On the other end of the ballpark spectrum, half a season in Petco Park (as well as a disproportionate number in parks such as Dodger Stadium and AT&T Park) drop Chase Headley‘s projection from .283/.353/.408 to .269/.342/.383.

I apologize for any inconvenience and welcome any comments from subscribers who suspect something may be amiss. Sometimes we’ll have an explanation, but other times we have been able to catch errors. As a result of Matt Swartz’s testing at FanGraphs of several projections, including Oliver, I am currently at work on some improvements on projecting ERA, as well as an existing project to be able to project a pitcher as either a starter or a reliever for those times when a player’s role changes.

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Comments

  1. Brian Cartwright said...

    For each team, I take a weighted mean of the park factors of all the ball parks they are scheduled to play in during the coming season.

    Those are multi-year factors, calculated from past season games, and split for right and left handed batters. For pitching projections, these are applied based on the expected percent of right and left handed batters faced, by whether the pitcher throws right or left.

    For the new park in Miami and the reconfigured Citi Field in New York, I do not have an estimate of their factors. At the beginning of this season, the factors for those parks will start at 1.00, but as more games are played the factors will approach their ‘true’ value, as the observed data is regressed to the mean of 1.00.

    With some recoding, it would be possible to seed the new parks with a value other than 1.00, allowing me to make an initial guess as to how each new park will play.

  2. evo34 said...

    Thanks.  How many past seasons are used to calc. the factors?  Would you mind posting a table with all the park factors you are using this season?

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