Quality Starts and Differential Luck

On July 11, Zack Wheeler gave the Mets a quality start by either definition – he pitched 6 2/3 innings and allowed only one run for a game score of 64. The  Mets managed to convert it into a win, which they’ve managed to do in 27 of their 46 wins thus far this year. Zack’s made 12 quality starts this year (by the sabermetric definition of a game score of 50 or more), but the Mets have managed to convert only 5 of them into Ws for Zack; the team is 7-5 in those games, while Zack himself is 5-2. That’s a far cry from the Giants’ freakish Tim Lincecum (9-0 in 12 quality starts) and the Angels’ Garrett Richards (10-0 in 15 quality starts). (The whole list of pitchers with quality starts so far is here.)

That got me thinking – which teams do the best at converting quality starts into wins? Which teams are the worst? What’s the relationship? I grabbed all of these numbers and put them together into a spreadsheet in order to play with them.

First, a quick review of terms: A cheap win is a pitcher win in a non-quality start. A tough loss is a pitcher loss in a quality start. “Luck” is whatever I happen to be measuring at the moment, but today ‘luck differential’ refers to the difference between the percentage of wins that are cheap and the percentage of losses that are tough; in other words, luck differential = 100*[(CW/W) – (TL/L)]. For an individual pitcher, these are fairly random occurrences – no pitcher in MLB today hits reliably enough to consistently earn himself cheap wins – but it seems that aggregating by team allows for the quality of batting to smooth out over a large number of games.

The Texas Rangers lead the league in this sort of luck differential, with 4 of their 38 wins coming cheaply for over 10% cheap wins but only 2 of their 55 losses tough (3.64); the Atlanta Braves have the worst luck differential in the league with a high proportion of tough losses (17/42, or 39.53%) and a low number of cheap wins (3/50, or 6%) for a total of -33.53. The Mets themselves convert less than 50% of their quality starts into wins for the starting pitcher.

These numbers are indicative of a general trend. The more quality starts a team has, the more negative its luck differential is (ρ = -.72 – an extremely strong correlation) and the more wins a team has, the more negative its luck differential is (ρ = -.20 – a bit weaker). Essentially, teams with more quality starts generate more wins (ρ = .56), regardless of the fact that sometimes they lose those quality starts, too. Surprisingly, the Mets have a -21.67 luck differential, one of the most negative in the league, probably due to the fact that they convert so few quality starts into wins.