About SNWL

by Michael Wolverton

Please feel free to email Michael with questions or comments at mjw@erg.sri.com.

The Support-Neutral pitching stats are designed to measure the value of a start in terms of how much it adds or subtracts from the team's chance of winning. Using situational scoring tables and some basic laws of probability, I calculate the probabilities that a pitcher's start will lead to a W or an L for him, as well as a win or a loss for his team. When totaled over all of a pitcher's starts, that gives us the three SN measures:

  • Support-Neutral Wins and Losses (SNW/SNL) -- a starter's expected W/L record, given the way he pitched in each game and assuming that he had league-average support from his offense and his bullpen.
  • Support-Neutral Value Added (SNVA) -- the number of games the starter is worth to an average team in the standings, over (or under) what a league average starter is worth.
Each of the three numbers is calculated separately for each individual start, and then summed to get seasonal totals. Looking at a starter's performance game-by-game like this removes distortions that can be introduced by looking at cumulative run prevention (e.g., ERA or Thorn and Palmer's Adjusted Pitching Runs). In particular, the SN stats recognize that a pitcher can only cost his team a single game in a single start, so it puts a limit on how much a single bad outing (e.g., 2 IP, 11 R) can hurt his season/career value.

This year there was a small but important change to the way the stats are calculated. Previously, the SN stats have been calculated based only on the number of innings pitched and the number of runs charged to a starter for each game. Now, the input will include the state of the bases when he leaves the game. So, for example, if a starter gets yanked with 2 outs in the 7th after 3 runs have scored with runners on first and third, and his reliever allows both runners to score, the SN stats will be calculated based on:

6 full innings, 3 runs in, runners on 1st and 3rd with 2 outs.

rather than the old way:

6 full innings, 2 outs, 5 runs allowed.

This new method provides results which are more "support-neutral", in that a starter's numbers will not be skewed by really good or really bad bullpen support in the inning he leaves the game.
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