Sunday, July 13, 2008

Baseball team stats and individual stats

In the course of making its funnies, Fire Joe Morgan applies statistics to baseball as well as just about anyone, but I think Ken Tremendous makes an interesting mistake in this takedown of the site's eponymous punching bag. Joe Morgan writes that the Red Sox are "the best team in the game," and KT replies, "The Cubs have a better team ERA and a better team OPS. For the record."

Leaving aside the question of whether a snapshot of team performance is a good way to call one team better than another, I want to focus on the use of ERA and OPS as measures of team performance. They are generally excellent measures of individual performance. The best quick justification of OPS, however, is that it basically explains where runs come from--team OPS correlates better with team runs than, say, batting average. And OPS scales well to the individual: one player's OPS gives you a pretty good sense of how much that player contributes per plate appearance to his team's scoring.

If we want to look at team performance, however, we can eliminate the middleman: it's all about the runs. Forget stats that correlate well with runs--use runs! And on the pitching side, we can drop the "earned" component of ERA, since the whole point of that is (however roughly) to separate individual from team performance. Again, use runs!

A recent Rob Neyer column about the Tampa Bay Rays touched on another case where statistics work fundamentally differently at the team and individual level. Neyer points out that the Rays have taken a huge step up in defensive efficiency this year:

[T]here's an incredibly simple statistic that tells us almost everything. Defensive Efficiency -- invented by Bill James in 1975 -- never really has caught on, which is sort of bizarre because it essentially answers a most basic question: "When a batted ball is put into play against a team, what percentage of the time does that team succeed in turning that ball into an out?"

In 2007, the Tampa Bay defense turned 66.2 percent of balls in play into outs. That figure was 30th best in the majors.

In 2008, the Tampa Bay defense has turned 72 percent of balls in play into outs. That figure is second best in the majors.

Defensive efficiency is like measures of individual fielding that attempt to discover a player's ability to convert balls hit near him into outs; at the individual level, such statistics are always beset by the difficulty of establishing the player's zone accurately. At the team level, aside from relatively minor park effects, the problem disappears: every team is responsible for the whole field. And for a single team playing in the same park, year-to-year comparisons become sublimely simple.

The moral of my story: sometimes the nuances of measuring individual performance cause us to overlook simple, powerful team statistics.