Using actual stats, Mariano's .47 HR/9 ranked a seemingly mortal 264th out of the 1037 pitchers who have pitched at least 1000 innings since 1901. However, since Mariano has pitched in the highest HR environment of all time, looking at adjusted stats underrates Mariano. For example, Hall of Famer Rube Marquard had a .291 career HR/9, ranking him well above Mariano. But Marquard pitched from 1908-1925, where the average HR/9 was .235. So Marquard was actually 20% worse than league average at preventing HRs, while Mariano's .47 HR/9 is more than twice as good at the league average from 1995-2011 (which was a 1.064 HR/9).
To see beyond such distortions, we need to adjust the pitchers' HR/9 stats to a common scale. In this exercise, I adjusted career HR/9 stats to the 1953 AL, which I chose because that league averaged a HR/9 of .715, which is almost identical to the MLB historical average HR/9 of .714. (Note: The method would rank pitchers the same regardless of which year and league used as the adjustment target.)
To adjust, I used the Home Runs Plus (HR+) statistic, which is one of the "Plus" statistics Diamond Mind Baseball uses to adjust for era differences. HR+ measures how far above or below average a given pitcher's HR/9 in a season was to the league average - a score of 100 is average, above 100 is above average, etc. For example, in 2011 Mariano's HR+ was 224, indicating he was 124% better than the 2011 AL league average in preventing home runs.
Once you have pitchers' annual HR+ stats, you can use that data to calculate pitchers' career HR+ stats. You can then use career HR+ stats to adjust pitchers HRs allowed and HR/9 for a given season's HR rate, which provides the needed common scale mentioned earlier. To spare those who are averse to math and are just interested in the results, the calculation method is described on a separate page - see http://www.guttygrittyyankees.com/p/adjusting-career-home-runs-allowed-hr9.html.
The following table lists the top 25 pitchers in MLB history in HR+, for pitchers with at least 1000 IP through 2011, and also adjusted HRs allowed and HR/9 to the 1953 AL. Mariano ranks 4th - his HR+ of 228 indicates he was 128% better than league average at preventing HRs.
|Pitcher||Debut||IP||HRs||HR/9||Expected HRs||HR+||Adjusted HRs||Adjusted HR/9|
I posted a spreadsheet listing these stats for all pitchers with at least 1000 IP - see https://docs.google.com/spreadsheet/ccc?key=0AkMu391-e5t0dDdlTjhaNlVpZWI1anBqVklWTlBLb3c.
There are many interesting things about the list. For example, apart from being exceptionally good at preventing HRs, what do Frank Corridon and Mariano Rivera have in common?