Understanding run differential
The most basic meaning of run differential is understood as the difference between the number of runs a team scores versus the number of runs they allow. The Chicago Cubs, for example, have scored a total of 200 runs this season, while allowing 146 runs. This gives them a total differential of +54.
According to Fangraphs, the Cubs currently sit in fourth overall in baseball, scoring 5.41 runs per game. The team has allowed the eighth-fewest runs per game, at just 3.95. At some point down the line, stat nerds figured out that roughly ten runs were equivalent to one victory. Fangraphs has a measure known as R/W or runs per win. In 2017 the R/W was 10.04, the highest in the last decade. This season, thus far, the R/W is 9.69. So, what does this all mean?
In an attempt to take this data and apply it to a projected number of wins over a season, you must start at the midway point, or .500. As the Cubs have played 37 games, halving it would give you 18.5, so roughly 19 wins. Making the run differential divisible by ten would project 5.4 more wins, giving them, approximately 24 wins. As it were, they are 21-16, meaning they are playing below their projections at this point.
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The 2016 season saw the Cubs finish with a run differential of +252. The team finished 103-59 that season, in route to winning the World Series. Strangely enough, the team played under their expected value. The Cubs expected win total that season was 106.
Unsurprisingly, the Cubs finished 2016 first overall in run differential average at 1.6. The next closest team, Boston Red Sox, finished 0.5 runs behind the Cubs.
Last season, the Cubs arrived back to earth finishing with a still impressive +127 run differential. They finished 92-70, which is not far off from their expected finish of 93-69. Unsurprisingly, the run differential average was down to 0.8. However, it was still a top ten finish. The two eventual World Series opponents, Houston Astros, and Los Angeles Dodgers finished tied for second respectively at 1.2.