Posted by Andy on August 30, 2010
This result isn't too surprising, but it turns out that there is a fairly strong correlation between average run scoring and percentage of games that go to extra innings. This makes intuitive sense--if league-wide run scoring is higher, it's less likely that both teams in a game will end up with the exact same number of runs at the end of 9 innings.
Click through for the data.
This data was easy to collect. I used two Team Batting Game Finders to find the total number of games each year as well as the total number of games of 9 innings or less. That gave me the fraction of extra-inning games. Then I picked off the run-scoring numbers from the MLB Batting Encyclopedia page.
Here's a scatter plot for all that data:
Each data point represents a single year. The average runs per game is on the x-axis and the fraction of games going to extra innings is on the y-axis. For example, this year, teams are averaging 4.42 runs per game and 9.1% of games have gone to extra innings. In 2009, runs were more plentiful at 4.61 per game and only 8.0% of games went to extra innings. In 1988, runs were scored only 4.14 per game and 9.4% of games went to extra innings. Back in 1965, run scoring was 3.99 per game and 11% of games went to extra innings.
The R-sqaured value of 0.27 tells us that there is a correlation coefficient of 0.52 between run-scoring and extra innings--that's a pretty strong link between the two.