October 5, 2017
Flu-Like Symptoms
The Bill James World Series Prediction System, Updated
by Rob Mains
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. The system from the 1984 Abstract was:
- Give the team with the better record one point for each half-game difference in won-lost percentage.
- Give three points to the team that scored the most runs.
- Give 14 points to the team that hit fewer doubles.
- Give 12 points to the team that hit more triples.
- Give 10 points to the team that hit more home runs.
- Give 8 points to the team with the lower batting average.
- Give 8 points to the team with fewer errors.
- Give 7 points to the team that had more double plays.
- Give 7 points to the team that allowed more walks.
- Give 19 points to the team that threw more shutouts.
- Give 15 points to the team whose ERA was more below the league average.
- Give 12 points to the team with most recent postseason experience. (In case of a tie, give the points to the team that had greater success.)
- For intraleague series, give 12 points to the team with the better head-to-head record.
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.
And now, with one more World Series, two more Championship Series, and four more Division Series under our belts, I was able to update the weights further. Here are the formulae for the 2017 postseason, using James’ original framework.
DIVISION SERIES
- Give 7 points to the team that scored fewer runs
- Give 10 points to the team with fewer doubles
- Give 15 points to the team with fewer triples
- Give 4 points to the team with the most home runs
- Give 12 points to the team whose batters had fewer walks
- Give 17 points to the team whose batters had fewer strikeouts
- Give 4 points to the team that had a higher on base percentage
- Give 4 points to the team that had a lower slugging percentage
- Give 8 points to the team that had fewer errors
- Give 27 points to the team that pitched more shutouts
- Give 8 points to the team whose pitchers had more strikeouts
- Give 8 points to the team with the lower ERA
- Give 12 points to the team with more recent postseason experience
CHAMPIONSHIP SERIES
- Give 12 points to the team that scored more runs
- Give 4 points to the team that hit fewer doubles
- Give 12 points to the team that hit fewer triples
- Give 13 points to the team that hit more home runs
- Give 14 points to the team with the higher batting average
- Give 10 points to the team with the higher on base percentage
- Give 10 points to the team with the higher slugging percentage
- Give 9 points to team that turned fewer double plays
- Give 10 points to the team whose pitchers had fewer strikeouts
- Give 6 points to the team whose pitchers allowed fewer walks
- Give 10 points to the team with the lower ERA
- Give 14 points to the team with better record
WORLD SERIES
- Give 10 points to the team that scored more runs
- Give 4 points to the team that hit more doubles
- Give 11 points to the team that hit more triples
- Give 3 points to the team that hit fewer home runs
- Give 4 points to the team with the higher batting average
- Give 4 points to the team with the higher slugging percentage
- Give 3 points to the team that committed fewer errors
- Give 10 points to team whose pitchers had more strikeouts
- Give 16 points to the team whose pitchers allowed more walks
- Give 3 points to team with the worse overall record
- Give 11 points to the team with the more recent postseason experience
And for the most part the weights worked pretty well. Looking at all postseason series since 1969, the year divisional play (and the Championship Series) began, the formulae above correctly predicted 69.0 percent of Division Series, 66.3 percent of Championship Series, and 73.9 percent of World Series.
As an example, our Playoff Odds Report gives the highest World Series win percentages to Cleveland (24.4) and Los Angeles (18.8). How might they fare?
- Cleveland scored 48 more runs. Ten points for Cleveland.
- Cleveland hit 21 more doubles. Four points for Cleveland.
- Cleveland hit 9 more triples. Eleven points for Cleveland.
- Cleveland hit 9 fewer home runs. Three points for Cleveland.
- Cleveland’s batting average was 14 points higher. Four points for Cleveland.
- Cleveland’s slugging percentage was 13 points higher. Four points for Cleveland.
- Cleveland committed 12 fewer errors. Three points for Cleveland.
- Cleveland pitchers had 65 more strikeouts. Ten points for Cleveland.
- Los Angeles pitchers allowed 36 more walks. Sixteen points for Los Angeles.
- Cleveland won two fewer games. Three points for Cleveland.
- Both teams were in the 2016 postseason, but Cleveland was more successful. Eleven points for Cleveland.
Cleveland, with 63 points, is favored over Los Angeles, with 16.
So what do they say for this year? I’ll give you every possible outcome, because why not.
DIVISIONAL SERIES
Cleveland favored over New York
Boston favored over Houston
Los Angeles favored over Arizona
Washington favored over Chicago
CHAMPIONSHIP SERIES
New York favored over Boston
Cleveland favored over Boston
Houston favored over New York
Houston favored over Cleveland
Washington favored over Arizona
Los Angeles favored over Washington
Chicago favored over Colorado
Chicago favored over Los Angeles
WORLD SERIES
Arizona favored over Boston
Chicago favored over Boston
Boston favored over Los Angeles
Washington favored over Boston
Houston favored over Arizona
Chicago favored over Houston
Houston favored over Los Angeles
Houston favored over Washington
New York favored over Arizona
Chicago favored over New York
New York favored over Los Angeles
Washington favored over New York
Cleveland favored over Arizona
Chicago favored over Cleveland
Cleveland favored over Los Angeles
Washington favored over Cleveland
So, per the system, the most likely outcomes are ...
DIVISIONAL SERIES: Cleveland and Boston advance, Los Angeles and Washington advance.
CHAMPIONSHIP SERIES: Cleveland defeats Boston, Los Angeles defeats Washington.
WORLD SERIES: I don’t want to jinx anybody.
Thanks to Dan Turkenkopf for research assistance. Actually, he helped last year, but I forgot to thank him then (sorry!), so I’m thanking him now.
Rob Mains is an author of Baseball Prospectus.
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2 comments have been left for this article. 
BOS > HOU, but HOU > NYY/CLE > BOS. got it.
Well, I could argue that, as the formulae imply, the factors that drive success in the DS are dissimilar from those in the CS. But you'd call me on that, wouldn't you? Hey, it's just what the formulae say.