Which Historical AL West Teams Have Been Luckiest?

Originally posted on September 1, 2014

One way of describing luck in baseball is by winning a lot more games than your runs scored/runs against (RS/RA) ratio should indicate. The Pythagorean estimator is probably the most famous way of predicting win/loss record based on RS and RA. Here it is:

Winning Percentage = RS^2 / (RS^2 + RA^2)

This isn’t the single most accurate estimator, but it’s pretty accurate. Looking at all of the MLB team-seasons since 2000, the Pythagorean estimator predicts win-loss record within 8 wins 95% of the time. It predicts win-loss record within 4 wins about 68% of the time.

One way to sharpen the pencil immediately is to adjust the exponent. I’ve done that for Oakland, Seattle, Texas and for the Los Angeles Angels (and their predecessor teams, the Anaheim Angels and California Angels) since 1990. In this set of data, the best-fitting exponent in the Pythagorean estimator isn’t 2; it’s 1.934. You probably don’t care about this paragraph, but I’m just saying it because it makes the results more robust.

Anyways, I was curious what the “luckiest” teams have been, amongst the “Core Four” AL West teams over the past 25-ish years. That’s why I am doing this.

Let’s look at the data.

There are 96 observations in this data set (four teams playing 24 seasons each, from 1990 through 2013). So, each team has one observation, for each season.

Here is a scatterplot of our 96 observations, plotting the Pythagorean-expected winning percentage (x-axis) versus the actual winning percentage (y-axis) for each team:

There are three diagonal lines in this plot. The middle one represents neither under-performance nor over-performance. In other words, if a team’s plot sits on the middle line, they won exactly as many games as Pythag predicted. If it lies above, they’ve overachieved, and if it lies below, they’ve underachieved. The top line is plotted at Pythag + .040, and the bottom line is plotted at Pythag – .040. In other words, if you’re above the top line, your team’s winning percentage exceeded the Pythagoran estimated wins by more than .040. These teams won 6-7 more games (4% of the season) than they “should have,” perhaps due to pure luck.  Similarly, if you’re near the bottom line, you lost 6-7 more games than you “should have.”

The moral of this story is that the 2008 Angels pretty much blow everybody away.  They’re an outlier. That team had 100 wins and 62 losses (.617 winning percentage). But based on runs scored versus runs allowed, you more reasonably would have expected them to win 88 games.

The plot in the far upper right is the 2001 Seattle Mariners, who won 116 games in the regular season.  According to Pythag, they overachieved, by about 6 games. Which is another way of saying that that team scored and prevented runs like a true 110-win team.  Pretty remarkable.