Which WNBA team had the most deceiving record in 2017?
The WNBA season is here and everyone is excited. As people make their predictions, they often start with a team’s record the previous season as a baseline. Then, they think about how changes since last season will impact its performance. They may not realize it, but that’s an example of Bayesian analysis — updating estimates based on new information.
Is last season’s record really the best starting point though? Probably not, especially in a league with just 34 games. Statistician Bill James first introduced to sports analytics the concept of Expected Wins based on scoring. He showed that in baseball, there is a predictable relationship between scoring and a team’s W-L record. It’s not a perfect relationship because of odd bounces and other factors, which can cause a team to have exceptionally good or bad records in a limited number of close games that don’t reflect its underlying abilities. However, scoring is still a better indicator of a team’s future performance than its actual W-L record is.
James called his approach Pythagorean Expectation because the formula for baseball resembled everyone’s favorite formula from geometry class, the Pythagorean theorem. We’ve adapted this approach for the WNBA to estimate Expected Wins, something we haven’t seen published much elsewhere (please let us know if you have!). Using data from the last eight years, the formula works out to be:
How well does it work? Here’s a graph of expected win percentages compared to actual win percentages for the last eight seasons. As mentioned above, it’s not perfect, but it’s reasonable given the length of the season. If the WNBA played 340 games instead of 34, we’d expect the relationship to be even stronger as the inconsistencies balance out.
So What?
Now we have the tools to see which teams had the most surprising record given their points scored and allowed last season.
The table makes clear that the Seattle Storm’s record of 15–19 was the most deceiving last year. Hopefully that’s not too controversial of a finding since they finished four games under 0.500 despite scoring and allowing exactly 2,808 points.
What does that mean for this season? The Storm weren’t as far away from contention last year as fans might think. By adding exciting rookie Jordin Canada to stalwarts Sue Bird, Breanna Stewart, and Jewell Loyd, they may be able to make a strong push in the 2018 playoffs.
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