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Are the Mariners Just Lucky or Actually Good?

A look at the Mariners run differential and their Cluster Luck.

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Houston Astros v Seattle Mariners Photo by Alika Jenner/Getty Images

Alright, it’s time to have the conversation. That’s right, it’s time to talk about cluster luck and the Mariners run differential. After their ugly loss yesterday afternoon, Seattle’s run differential fell to -58 on the year despite an actual record that sits seven games over .500. So with just a day left before the July 30 trade deadline and the Mariners in the thick of the AL Wild Card race, let’s talk about whether they’ve been extremely lucky or if they’re actually good.

Every year there are run differential overperformers and underperformers. This year, the Blue Jays have gone 50-48 despite a fantastic +85 run differential. That’s a .072 difference between their actual record and their expected record. The Mariners have outperformed their expected winning percentage by .087; that’s one of the largest differences between winning percentage and expected winning percentage in the last century.

Win% Anomalies (1900 – 2021)

Team Year Win% Expected Win% Difference
Team Year Win% Expected Win% Difference
Yankees 1931 0.614 0.705 0.091
Mets 1993 0.364 0.455 0.090
Athletics 1955 0.409 0.319 0.090
Reds 1981 0.611 0.523 0.088
Mariners 2021 0.534 0.447 0.087

The 2018 Mariners — another team that seriously outperformed their expected winning percentage — ended that season with a .074 difference between their actual record and their expected record. You could say they’ve been lucky to win nine more games than expected based on their run differential alone.

Relying on run differential alone is a simplistic way of discerning true talent. It’s useful for broadly describing team performance, but blowout wins and losses have a way of skewing the data a bit. Here’s a histogram showing the Mariners run differential per game this year.

The Mariners have been blown out in nearly as many 5-run-or-more losses as they have won close one-run games. That has a pretty sizable effect on their overall run differential. All those blowouts have skewed the Mariners run differential and they don’t have many blowout wins to offset those huge losses.

Another way of analyzing a team’s true talent is to turn to a concept known as cluster luck. Using the BaseRuns formula to estimate run scoring based on the number of baserunners, how many of those baserunners advanced, and the number of outs created, we can estimate the number of runs scored and allowed. Then we can compare actual results to come up with a tangible measure of how lucky a team has been.

Below is a line graph and table showing the Mariners actual run differential, expected run differential, and Cluster Luck this season.

Mariners Cluster Luck

Month Run Differential Expected Run Differential "Luck" Offensive "Luck" Pitching "Luck"
Month Run Differential Expected Run Differential "Luck" Offensive "Luck" Pitching "Luck"
April 1 2.50 -1.50 5.61 -7.11
May -43 -36.38 -6.62 -1.93 -4.69
June -5 -4.99 -0.01 -0.88 0.88
July -11 -26.34 15.34 5.32 10.02
Overall -58 -65.22 7.22 8.12 -0.90

Through the first three months of the season, the Mariners were actually a touch unlucky, with BaseRuns believing their run differential should have been around eight runs better than it was. That’s in large part due to a terrible month of May where the Mariners were no-hit twice and had seven blowout losses. But that bad luck through the first three months of the season pales in comparison to the huge amounts of good luck they’ve enjoyed in July. Despite ending the month with a -11 run differential and a 13-9 record, BaseRuns thought their run differential should have been 15 runs lower.

Just looking at the way this team is built confirms what this run differential and cluster luck analysis shows us. A fantastic bullpen gives the Mariners a shot to win any close game they’re playing in but a poor starting rotation means they’re prone to some really lopsided losses. Their offense is heavily reliant on hitting home runs to score, which means they can win those close games with a single hit but have trouble stringing together a bunch of hits to create those huge crooked scores in their favor.

The other day, Isabelle wrote about how the Mariners are one of the most clutch teams in major league history. That’s a big reason why they’ve been able to outperform their expected run differential. When they’re batting without any runners on base, their team wRC+ is an anemic 76, 29th in baseball. As soon as a runner reaches, that mark jumps up to 114 (5th) and even higher with a runner in scoring position (125 | 3rd). The same kind of trajectory applies to high leverage opportunities.

Mariners Performance by Leverage

Leverage wRC+ FIP
Leverage wRC+ FIP
High Leverage 138 (1st in MLB) 3.85 (8th in MLB)
Medium Leverage 98 (17th) 4.09 (13th)
Low Leverage 80 (29th) 4.43 (18th)

The Mariners have thrived when the game is on the line. That explains why they’ve been able to outperform their expected runs scored by eight runs. And with Rafael Montero removed from the back end of the bullpen and the emergence of Paul Sewald and JT Chargois, the bullpen has improved by leaps and bounds, helping them outperform their expected runs allowed by nearly 11 runs during the last two months.

So what does this all mean moving forward? Continued performance in high leverage situations or with runners on base can’t be assumed based on past results. Could the Mariners continue to squeak out a bunch of close wins down the stretch? It’s certainly possible. Are they as bad as their poor run differential paints them? Probably not. They do have 55 wins in the bank and any analysis looking back won’t change those results. They’re just two games behind the A’s for the second Wild Card spot and a few key upgrades to the roster could change the fortunes of the club in the second half.

Their results in June might paint the best picture of what this team is truly capable of during the last two months of the season. They were essentially luck neutral that month and finished two games over .500 with a -5 run differential. Extrapolating that kind of performance over their remaining schedule gets them to 87 wins. Because they were lucky enough to post a strong record in July against key opponents, they’re in a position to try and go for it. With a little more luck, and a bit more Chaos Ball, they could make things really interesting in late September.