Mains: Reviving Bill James' World Series prediction tool

From SABR member Rob Mains at Baseball Prospectus on October 4, 2018:

A couple of years ago, I dusted off Bill James’s World Series Prediction System, which he developed in 1972 and introduced to a wider audience in Inside Sports in 1982 and the 1984 Bill James Baseball Abstract.

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James calculated the weights by considering every postseason series to date, 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-AVG offenses might be dependent on stringing together a lot of singles and doubles but scoring in the postseason often depends on the long ball.

I updated the system in 2016 and 2017; James didn’t have data from 31 World Series and 62 League Championship Series that I had that 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, League or World). So I developed different formulae for each series type and ran another update in 2017.

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This page was last updated October 4, 2018 at 12:49 pm MST.