Bill James released a study on the Pythagorean theorem of baseball, looking specifically at teams that over perform their record. His paper on the study can be found in the comments section.
The Mariners this season had a ridiculously high difference between their actual wins and their Pythagorean wins. Even higher was the Arizona Diamondbacks (9th highest overperformance in baseball history). Fans are fond of saying that the local 9 will continue to outperform their pythag for whatever reasons (reliever, clutch hitting, William F. Bloomquist), and everybody is sure they will, but will they?
I’ll make it quick, the answer… kind of. James looked at the top 100 over and underperforming teams in history. He compared their over/underperformance of year 1 with the next year. The results: Teams that were in the top 100 next year DID over perform their Pythagorean record(the Mariners’ over performance is #28 in baseball history), although by only 1/2 a win. These are the teams that outperformed by the most and they only outperform by ½ win. Whatever magic they had didn’t seem to carry over. Teams that underperformed would do so again, but by an even smaller margin. For all teams in baseball history the correlation coefficient between year 1 and year 2 was .04. Negative 1 would represent a perfectly negative relationship (as one went higher, the other went lower), 1 would represent a perfectly positive relationship (as one went higher, the other went higher). .04 essentiallly means it is pretty random and over/underperforming one year had almost no relationship with over/underperformance last year. Teams just do not possess that skill.
On an interesting note, when comparing a team that overachieved with a team that didn’t overachieve, but had a similar RS/RA ratio, the team that overachieved performed better than the other by about 3.5 games.
Conclusion: It could go either way. The Mariners are far from a lock to beat their pythag again. There is about a 50% chance they will do better than expected next year, but there is a 50% chance they won’t. Teams have not shown the ability to consistently outperform their runs ratio. Going forward, we will need to look at the Mariners as what they are. They are a 79 win club that needs to add at least 10 wins to consider themselves playoff contenders.