Quote:
Originally Posted by jerrycapo
So I can learn a little more, you say OPS+ is league adjusted. How is that adjustment made and is that adjustment applied over different era's.
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This is from the batting glossary page on baseballreference.com - "how the adjustment is made". It can be applied to any player in any year, AFAIK.
Adjusted OPS+
This value is calculated differently from the Total Baseball PRO+ statistic. I chose OPS+ to make this difference more clear. PRO+ as best I can tell is
PRO+ = 100 * ( OBP/lgOBP + SLG/lgSLG - 1)/BPF
Where lgOBP and lgSLG are the slugging and on-base percentage of a league-average player, and BPF is the batting park factor. This takes into account the difference in runs scored in a team's home and road games, so it doesn't depend on how good an offense or defense a team has.
My method is slightly more complicated, but I think it is more correct. The BPF is set up for runs and the way it is implemented in PRO+ applies it to something other than runs.
- My method
- Compute the runs created for the league with pitchers removed (basic form) RC = (H + BB + HBP)*(TB)/(AB + BB + HBP + SF)
- Adjust this by the park factor RC' = RC*BPF
- Assume that if hits increase in a park, that BB, HBP, TB increase at the some proportion.
- Assume that Outs = AB - H (more or less) do not change at all as outs are finite.
- Compute the number of H, BB, HBP, TB needed to produce RC', involves the quadratic formula. The idea for this came from the Willie Davis player comment in the Bill James New Historical Baseball Abstract. I think some others, including Clay Davenport have done some similar things.
- Using these adjusted values compute what the league average player would have hit lgOBP*, lgSLG* in a park.
- Take OPS+ = 100 * (OBP/lgOBP* + SLG/lgSLG* - 1)
- Note, in my database, I don't store lgSLG, but store lgTB and similarly for lgOBP and lg(Times on Base), this makes calculation of career OPS+ much easier.