Mains: The Bill James World Series prediction system, updated

From SABR member Rob Mains at Baseball Prospectus on October 5, 2017:

Last year, I dusted off Bill James’ “World Series Prediction System,” which James developed in 1972 and introduced to a wider audience in Inside Sports in 1982 and the 1984 Bill James Baseball Abstract.


He calculated the weights by considering every postseason series and checking how often the winning team exhibited certain characteristics. Shutouts got a weight of 19 because among the series he considered, the team with more shutouts won 19 more times than it lost. The team with the fewer doubles won 14 more times, the team with the lower relative ERA won 15 more times, etc. And there’s an element of intuitive sense; high-average offenses may be dependent on stringing a lot of singles and doubles together, while scoring in the postseason is often long-ball dependent.

I updated the system last year. James didn’t have data from 31 World Series and 62 Championship Series that I had last year. And the Division Series didn’t even exist until 1995. I found two significant changes by compiling the data since the 1984 Abstract. First, unsurprisingly, the weights have changed. Second, I found that the weights differed depending on the type of series (Division vs. Championship vs. World). So I developed different formulae for each series type.

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Originally published: October 5, 2017. Last Updated: October 5, 2017.