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October 9, 2003

Getting PADE

Improving on Defensive Efficiency

by James Click

Evaluating defense has always been one of the more difficult tasks for performance analysts. The first reason for this is that looks can be deceiving. Sure, that acrobatic shortstop playing in the country's largest market might appear to be a superior defender to the untrained eye, but all too often we draw our conclusions by putting emphasis on the outcome rather than the process of fielding the ball, itself. The second reason is the still-severe limitations we face with regard to collecting data, and how to properly interpret that data once we get a meaningful amount of it. Granted, there are some statistics that can be used when evaluating defense--errors, fielding percentage, Range Factor, Zone Rating, etc.--but none of them is without its flaws.

Which bring us to one of Bill James' measures for quantifying defensive performance: Defensive Efficiency (provided here by Keith Woolner). Defensive Efficiency is a metric that measures a team's ability to turn balls-in-play into outs, using the formula...

(TotalOuts - Strikeouts)/(BIP-HR)

Despite being raw and only applying to entire teams, Defensive Efficiency is a fair measure of overall defensive performance. But that doesn't mean it can't be improved.

Defense can be broken down into several facets, primarily pitching, ballpark, and actual defensive performance. While we've conceded that pitching and defense are extremely difficult to separate, it's much easier to take into account the venue in which the game took place. For example, looking at this season's numbers, Defensive Efficiency rates the Rockies as one of the league's worst defenses, while the Dodgers have one of the best. But how much of that is actual performance, and how much of that is simply a function of each team's playing environment? Can we determine how the Rockies would perform if they played someplace else?

We already use park factors when adjusting hitting and pitching statistics, and they can be applied to defense as well. However, using established park factors to adjust our defensive statistics would yield skewed results, as they take into account the full slate of offensive statistics, most notably home runs. Smaller ballparks are the main concern, as they yield a higher park factor, mostly thanks to home runs. But the fact of the matter is that many small ballparks might actually be easier to play defense in, since their outfield is much smaller.