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February 23, 2004

Baseball Prospectus Basics

The Support-Neutral Stats

by Michael Wolverton

"...and the tough-luck loser in tonight's game is..."

We hear the above quote in dozens of post-game wrap-ups every year. A starting pitcher goes seven or eight innings and gives up only one or two runs, but his team's offense can't produce anything, so he gets stuck with an "L" next to his name in the box score. The fact that "tough-luck loser" is such a commonly invoked cliche suggests that it's widely recognized that the "L" isn't doing a very good job of measuring the starter's contribution, at least in those situations. But that still doesn't stop the W/L record from being possibly the most prominently used statistic to evaluate starting pitchers in major media baseball coverage.

The idea behind the pitcher's W/L record is flawed on its face. Wins are a team thing, after all, not a pitcher thing. If the offense fails to put runs on the board, or if the bullpen melts down in the late innings, the starter won't get the win no matter how well he pitches. Conversely, if the offense is having a great night (or if they're going up against the Rangers, which is pretty much the same thing), the starter doesn't have to do anything more than last five innings to get the W.

And it's not like the luck always evens out over the course of a season. Just ask any member of last year's Tigers rotation. Even over a long career, offense and bullpen support be a significant distortion to a pitcher's W/L record. Just ask Lew Burdette. Or Bob Friend.

The pitcher's role is pretty much limited to keeping as many runs off the board as possible. That may sound obvious, but it goes against the notion that "pitching to the score" is an important part of a pitcher's job. Plenty of people have looked for a significant ability to pitch to the score without finding it (most notably Greg Spira). And Bill James found that this year's ERA is a better predictor of next year's W/L record than this year's W/L record is.

Toward that end, there's been a gradual (very gradual) movement among baseball fans over the past 20 years to pay less attention to W/L record and more to ERA in pitching evaluation. And that's a good thing, since ERA is a good statistic. It's measuring more of what it's supposed to be measuring--the performance of the pitcher--and less of the performance of his teammates.

Still, as good a statistic as ERA is, it's not without its limitations. Some of those limitations are:

  • It doesn't take into account the scoring environment--park, league, and era--in which the pitcher is performing. A 3.00 ERA in Coors Field in 2000 is radically different from a 3.00 ERA in Dodger Stadium in 1968.

  • The rule for eliminating "unearned" runs is silly, and it only serves to remove an arbitrary subset of the runs the pitcher gave up.

  • Since ERA is a rate stat, it doesn't incorporate playing time. It's easy enough to conclude that a 3.00 ERA over 180 innings is better than a 3.00 ERA over 120 innings. But how do we compare 180 innings of a 3.50 ERA to 120 innings of a 3.00 ERA?

  • It includes the runners allowed in by the starter's bullpen successors. Really good bullpen support--i.e., stranding a lot of inherited runners--can chop a quarter or even a half a run off a starter's ERA. Really bad bullpen support can add about as much.

  • It treats all runs as equal. More on this in a second.

To address these limitations, I developed the Support-Neutral Win-Loss (SNWL) record, which we've been tracking at Baseball Prospectus since the 1997 book. (Click here to see the final numbers for 2003.) The idea is to measure starting pitchers' performance on the familiar and understandable W/L scale, but without the distortions of run support and bullpen support that plague the traditional W/L record.

A starter's SNWL record is his expected (in the statistical sense) W/L record--how many games he would be expected to win and lose given his pitching performances, assuming he had a league average offense and bullpen behind him. An additional statistic, Support-Neutral Value Added (SNVA), measures the total number of extra games his team would be expected to win with his pitching performances instead of an average pitcher's.

The calculations are adjusted for park and league scoring level, they're based on runs allowed rather than earned runs, and they're based on the situation in which the starter leaves the game (before, not after, his relievers finish with the runners he turns over). So they're not subject to the problems with ERA we noted above. I won't go into the calculations in this article, but you can get all the gory details here.

One other benefit of the Support-Neutral numbers is that they look at each start's contribution to winning individually rather than a season's run total cumulatively, so a single disastrous outing can't have the disproportionate impact that it can have on a starter's ERA. Consider a simple example:

                    Start 1         Start 2
		 ----------       ---------
Pitcher A        0 IP, 10 R       8 IP, 0 R
Pitcher B        4 IP,  5 R       4 IP, 5 R

Their ERAs are equal, but Pitcher A's starts are likely to lead to more wins than Pitcher B's. An average team has a good chance of going 0-2 behind Pitcher B's two starts, but that same team is almost guaranteed to win Pitcher A's second game. The Support-Neutral stats account for the fact that the 10 runs concentrated in Pitcher A's one start don't do the same amount of damage as the ten runs spread among Pitcher B's two starts. The above-linked article covers this in more detail.

Are the Support-Neutral stats the be-all and end-all in assessing pitching performance? Of course not. There are many different goals for pitching evaluation, and many different tools that are useful for achieving those goals. If you're doing short-term prediction, for example, you're certainly better off looking at the components of run scoring rather than a stat based on runs allowed itself. And we haven't even touched the nasty question of separating pitching from fielding.

But if you're looking at a pitcher's contribution toward winning, the Support-Neutral numbers have a lot going for them. They correct for the big distortions of the traditional W/L record, and the small distortions of ERA.

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