This is a guest post by Clem Comly and Tom Ruane, based on recent posts to SABR-L.
2010 Giamts ERA for Sept./Oct. regular season was 1.91. Cy Morong asked me off-list how unusual is a montly ERA that low.
So I asked baseball-reference.com's Play Index. Unfortunately, I couldn't specifically ask about the best team ERAs for calendar months.
So I changed the question (perhaps inspired by the Kobyashi Maru). I decided to look at ONLY September combined with October. Baseball-reference.com couldn't answer that question directly, but I asked it to round up the usual suspects That is, I asked for lowest OR of exactly 26 games for a team at the end of its season.
For those teams after 1919, I could manually look up the monthly splits for those teams on the Retrosheet site (and where necessary combine Sept. and October splits to calculate ERA for Sept./Oct.
It turns out the Giants finished with an extremely good but not record-setting autumn. Based on the results of the query, the odds are very good that the record holder for 1920-2010 is 1965 Dodgers at 1.50. I suspect if one worked through 1901-1919 a team from that era with easier unearned run rules and more errors would have the record. I include the 26-game stats (date range, game sequence number range, and OR in the span) below to indicate how much of the Sept./Oct. period the query covered. Also, the speed with which the OR zoomed up indicates the actual record holder is the 1965 Dodgers (a team that didn't make the list would have at least 65 OR in 26 games while '65 LA only had 47, only 72% of 65).
B.Retro means indicated season before the first season chronologically that Retrosheet has monthly splits. Retrosheet has splits for 1920-2009. 2010 data from baseball-reference.com.
Here is what Tom Ruane came up with after Clem raised the issue
[Editor's note: I added the relative ERA figures and they are not necessarily the 10 lowest ever, but all of them are probably near the top. The NL ERA in Sept/Oct this year was 3.72. That gives the Giants a relative ERA of .513, which looks very good by historical standards]
Clem Comly is the vice-president of Retrosheet and co-chair of SABR's statistical analysis committee while being a member of several other committees. He is also a Phillies fan.
Tom Ruane, a computer programmer in Poughkeepsie, N.Y., is a member of Retrosheet's board of directors. He has published articles in "The Baseball Research Journal" and "By The Numbers." He won SABR's highest honor, the Bob Davids Award, in 2009.