These numbers are difficult to calculate and I would refer you to acopy of Total Baseball if you wish to recreate the park factor values.

A value above 100 indicates a park good for hitters,and below 100 indicates a park good for pitchers. The ERA+ and PRO+ valuesare adjusted to both the league average and the park the pitcher orbatter played in.

The career totals are gathered by finding what a league averageplayer would have done given the same playing time as the player inquestion and then summing these values up over the player's career.Given how we store the seasonal data (as an ER total not the leagueERA) it is very easy to calculate. Similarly for PRO+ (league timeson base and league total bases).

Note that the lg_ERA and lg_OPS values are for a league averageplayer in that ballpark for single season data, and for a leagueaverage player with the same career path as the given player. Thismeans that two players from the same league will have different valueshere if they played in different parks.

Calculation of Park Factors

WE largely follow the method spelled out below. Historically, B-Rhas been using single-year park factors for recent years and 3-yearpark factors historically. We have changed that to now use 3-yearfactors by default for all years. Of course, the current season isonly really a 2-year factor. The current year and last year. This canlead to some big changes in the numbers, from what had been on thesite.

Two other major differences.

1) Interleague games are not used in thecalculation. They really mess things up because in some games theteams have the DH and in others they don't. These series are alsotypically not home-and-home series.

2) For the years 1957-present, we use runs per 27 outs ratherthan runs per game (the IPC is always 1.00 in these cases). This ismore accurate than using IPC.

Overall, these two changes make only small changes to the numbers,but we believe they are more accurate this way.

This information is taken from anArchivedCopy of the now defunct TotalBaseball.com website. THIS DOES NOTBELONG TO US AND MAY BE REMOVED IF ASKED TO DO SO BY AREPRESENTATIVE OF TOTALBASEBALL.COM.

PARK FACTOR Calculated separately for batters andpitchers. Above 100 signifies a park favorable to hitters; below 100signifies a park favorable to pitchers. The computation of PF isadmittedly daunting, and what follows is probably of interest to themerest handful of readers, but we feel obliged to state themathematical underpinnings for those few who may care. We use athree-year average Park Factor for players and teams unless theychange home parks. Then a two-year average is used, unless the parkexisted for only one year. Then a one-year mark is used. If a teamstarted up in Year 1, played two years in the first park, one in thenext, and three in the park after that and then stopped play, theaverage would be as follows (where Fn is the one-year park factor foryear n):

   Year 1 and 2 = (F1 + F2)/2    Year 4 = (F4 + F5)/2     Year 3       =  F3            Year 5 = (F4 + F5 + F6)/3                                   Year 6 = (F5 + F6)/2

Step 1. Find games, losses, and runs scored and allowed for eachteam at home and on the road. Take runs per game scored and allowed athome over runs per game scored and allowed on the road. This is theinitial figure, but we must make two corrections to it.

Step 2. The first correction is for innings pitched at home and onthe road. This is a bit complicated, so the mathematically faint ofheart may want to head back at this point. First, find the team's homewinning percentage (wins at home over games at home). Do the same forroad games. Calculate the Innings Pitched Corrector (IPC) shownbelow. If it is greater than 1, this means the innings pitched on theroad are higher because the other team is batting more often in thelast of the ninth. This rating is divided by the Innings PitchedCorrector, like so:

     (18.5 -- Wins at home / Games at home)  IPC = -----------------------------------       (18.5 -- Losses on road / Games on road)

Note: 18.5 is the average number of half-innings per game if thehome team always bats in the ninth.

Step 3. Make corrections for the fact that the other road parks'total difference from the league average is offset by the park ratingof the club that is being rated. Multiply rating by this Other ParksCorrector (OPC):

            No. of teams  OPC=----------------------------------       No. of teams - 1 + Run Factor, team

(Note that this OPC differs from that presented earlier in TheHidden Game of Baseball, for in preparing the pre-1900 data for TotalBaseball, we discovered that for some parks with extremecharacteristics, like Chicago's Lake Front Park of 1884, which had aHome Run Factor of nearly 5, the earlier formula produced wrongresults. For parks with factors of 1.5 or less, either formula workswell.)

Example. In 1982, Atlanta scored 388 runs and allowed 387 runs athome in 81 games, and scored 351 and allowed 315 on the road in 81games. The initial factor is (775/81) / (666/81) = 1.164. The Braves'home record was 42-39, or .519, and their road record was 47-34, or.580. Thus the IPC = (18.5 - .519) / (18.5 - .420) = .995. The teamrating is now 1.164/.995 = 1.170. The OPC = (12) / (12 - 1 + 1.170) =.986. The final runs-allowed rating is 1.170 X .986, or 1.154.

We warned you it wouldn't be easy!

The batter adjustment factor is composed of two parts, one the parkfactor and the other the fact that a batter does not have to face hisown team's pitchers. The initial correction takes care of only thesecond factor. Start with the following (SF = Scoring Factor,previously determined [for Atlanta, 1.154], and SF1 = Scoring Factorof the other clubs [NT = number of teams]):

    SF - 1  1 - -----      NT - 1

Next is an iterative process in which the initial team pitchingrating is assumed to be 1, and the following factors are employed:

RHT, RAT= Runs per game scored at home (H) and away (A) byteam,

OHT, OAT= Runs per game allowed at home, away, by team

RAL = Runs per game for all games in the league.

Now, with the Team Pitching Rating (TPR) = 1, we proceed tocalculate Team Batting Rating (TBR):

    |RAT   RHT| |   TPR-1|      |--- + ---| |1+ -----|      |SF1   SF | |   NT- 1|  TBR=------------------------------               RAL

    |OAT   OHT| |   TBR-1|      |--- + ---| |1+ -----|      |SF1   SF | |   NT- 1|  TPR=------------------------------               RAL

The last two steps are repeated three more times. The final BattingCorrector, or Batters' Park Factor (BPF) is

      (SF + SF1)  BPF=----------------      |    |  TPR-1 | |      |2 X |1+ -----| |      |    |   NT-1 | |

Similarly, the final Pitching Corrector, or Pitchers' Park Factor(PPF) is

      (SF + SF1)  PPF=----------------      |    |  TBR-1 | |      |2 X |1+ -----| |      |    |   NT-1 | |

Now an example, using the 1982 Atlanta Braves once again.

      388                  351        RHT - --- - 4.79     RAT - --- - 4.33        81                   81

      387                  315        OHT - --- = 4.78     OAT - --- = 3.89        81                   81

      7947                          RAL = --- = 8.18     NT   = 12        972

                       |1.154-1|  SF = 1.154       SF1=1-|-------|=.986                         |   11  |

    |4.33  4.79| |   1- 1|      |--- + ----| |1+ --- |      |.986 1.154| |    11 |  TBR=---------------------------=1.044               8.18

    |3.89  4.78| |   1.044- 1|      |--- + ----| |1+ --------|      |.986 1.154| |    11     |  TBR=---------------------------=.993               8.18

Repeating these steps gives a TBR of 1.04 and a TPR of .97. TheBatters' Park Factor is

      (1.154 + .986)  BPF=----------------=1.07      |    |  .99-1 | |      |2 X |1+ ---- | |      |    |    11  | |

This is not a great deal removed from taking the originalratio,

    1.170 + 1      --------- , which is 1.08        2

The Pitchers' Park Factor may be calculated in analogousfashion.

To apply the Batters' Park Factor to Batting Runs, one must usethis formula:

For example, if a player produces 20 runs above average in 700plate appearances with a Batters' Park Factor of 1.10, and the leagueaverage of runs produced per plate appearance is .11, this means thatthe player's uncorrected Batting Runs is 20 over the zero point of 700X .11 (77 runs). In other words, 77 runs is the average runcontribution expected of this batter were he playing in an averagehome park. But because his Batters' Park Factor is 1.10, which meanshis home park was 10 percent kinder to hitters (than the average), youwould really expect an average run production of 1.1 X 77, or 85runs. Thus the player whose uncorrected Batting Runs is 97 with a BFof 1.1 is only +10 runs rather than +20, and 10 is his Park AdjustedBatting Runs (in the Player Register, BR/A):

10 = 20/1.10 - .11 X (1.10 - 1) X 700.