As if to put an exclamation point on their recent winning ways, the Toronto Blue Jays went into the Bronx over the weekend and swept the AL East-leading New York Yankees by a combined score of 10-1, narrowing New York’s division lead to a mere game and a half.
The Blue Jays’ hot streak has been linked to their aggressive deal-making before MLB’s July 31 trade deadline. But as Victor Mather of The New York Times rightly pointed out in a column Monday, Toronto had the characteristics of a dominant team before they added any players. As of July 28, they possessed the American League’s top per-game run differential and its best underlying statistical fundamentals, despite a sub-.500 record.
Baseball statheads have long known that a team’s run differential is generally a better predictor of future wins and losses than its record. So, it stood to reason that the Blue Jays would eventually turn things around. But we should also be careful not to swing too far in the opposite direction and ignore team records entirely.
Run differential — which is usually expressed on the same scale as a winning percentage using the pythagorean formula — is indeed a better predictor than a team’s actual record. When predicting future wins at this stage of the baseball season, run differential is about 1.8 times as important as actual winning percentage. But that still means the optimal mix of pythagorean and actual winning percentages is roughly a 65-35 split, a ratio that gets closer to 50-50 as the season draws toward its end.
This means that even if you know a team’s run differential, it also pays quite a bit to know its W-L record. While teams do tend to regress toward their underlying metrics, especially early in the season, we can capture additional signal by measuring a team’s ability to actually win ballgames (shock!).
As for the Blue Jays, they’re clearly one of the AL’s best teams, especially with the extra weapons they picked up at the deadline. But their recent run of success wasn’t completely fated by their great underlying metrics earlier in the season — odds are, that disappointing record also said something real about them.