One way of normalizing pitchers' HR/9 is to leverage the HR-Plus (HR+) statistic that Diamond Mind Baseball uses to adjust for era differences. After calculating pitchers' career HR+, you can then use that to calculate adjusted career HRs and HR/9.
I tend to doubt I am the first person who has done this; however, after a fair amount of searching, I could not find a writeup on how to make this career adjustment. So this page documents how used HR+ to calculate career HRs allowed and HR/9 normalized to a given league, in case anyone else is interested.
The formula to calculate a pitcher's HR+ in a given season works as follows:
- Calculate the number of HRs a league-average pitcher would have given up in the pitcher's innings pitched. Do so by multiplying the league average HR/9 and the pitcher's innings pitched, then divide by 9. Let's call the result "Expected HRs".
- Divide Expected HRs by the pitchers actual HRs, then multiply the quotient by 100. Values indicate the pitcher is below average, values above 100 indicate above average.
- Expected HRs: (.987 * 61.333)/9 = 6.726186.
- HR+ (rounded): (6.726186/3) = 224.
To calculate career HR+ and adjusted HR/9 stats is then a matter of doing the following:
- Calculate the pitcher's Expected HRs for each season.
- Calculate career HR+: Divide the sum of the pitcher's expected HRs by the pitcher's career totals, then multiply by 100.
- Calculate adjusted HR/9 as follows:
- Pick the target HR/9 environment to adjust to. I chose the 1953 AL, with a .715 HR/9.
- Calculate the number of HRs the pitcher would have given up if his entire career occurred in the target league. This is done by multiplying the target league's HR/9 by the pitchers innings pitched, then dividing the product by 9. Let's call this "Expected Adjusted Career HRs."
- Divide the Expected Adjusted Career HRs by the career HR+, then multiply the quotient by 100. These are the "Adjusted Career HRs."
- Divide the Adjusted Career HRs by the career innings pitched, then multiply by 9.
- Career expected HRs: the sum of his season's Expected HRs is 148.65.
- Career HR+: (148.65/65) * 100 = 228.692.
- HR/9 = .313 (rounded), calculated as follows:
- Expected Adjusted Career HRs: (.715 * 1211-1/3) * 9 = 96.2.
- Adjusted Career HRs: (96.2/228.692) * 100 = 42.08.
- Adjusted HR/9: ( 42.08/1211-1/3) * 9 = .313.