Saturday, 12 July 2008

A Defensive Metric

Last night, I was trying to explain, for the first time in public, to a couple of guys I know, my fielding metric for pre-Zone Rating players. I don't think it went very well, so I'll try again here.

You need to go back to Chris Dial's method of converting Zone Rating into Runs Saved, which is the foundation for what I do. In the modern era of Play-By-Play accounts, it's possible to create a Zone Rating to show how many balls hit into a player's fielding zone are turned into outs. However, before 1987 we don't have this information, so we have to figure out a way of estimating it.

Chris's research revealed that there is an average number of fielding chances at a position. For example, at shortstop the average team has 532 chances per season of 1440 innings. This number, I'm told, is apparently relatively static over the seasons. Chris also established a Run Value for each chance. Thus, an average team's shortstops save about 400.6 runs per season.

Since we have total values for all those who fielded at shortstop for Major League teams going way back in the history of Organized Baseball, we can, with a degree of confidence based on the recording of other information, estimate a Zone Rating for each position on a given team in a given season. And from that, we can work out how many runs the whole season's worth of players at a given position had saved.

I'll give a detailed example in the next post.

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