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April 28, 2014

Moonshot

How Quickly Do Team Results Stabilize?

by Robert Arthur


With the end of April looming, we can begin to shed some of our fears regarding small sample size. Statistics like strikeout and walk rates have passed critical thresholds on their march toward stabilization, and so we are beginning to get a first look at how well individual players will perform. The requisite early-season loss of ~20% of each team’s starting rotation to the failure of a certain crucial ligament has taken its toll, resulting in a clearer picture of who will make each team’s starts.

All of which is to say, we can begin to turn our attention to matters larger than individual players. Since the ultimate goal of every team is to win a championship—and the best way to win a championship is simply to field a very good team—the question of utmost importance is simply: How good is my team?

In light of this question, I examine here how quickly team quality stabilizes over the course of a season. At a fundamental level, good teams are defined by 1) scoring lots of runs, and 2) not allowing the other team to score many runs. Therefore, I take as my measurements of quality runs scored per game and runs allowed per game.

While there is a simple relationship between the number of runs scored/allowed and wins (via the Pythagorean expectation), that relationship is quite noisy. First and foremost, the noise results from sequencing, or the luck a team has in apportioning its runs to individual games. A bad team may thus end the season with an excellent record and a playoff berth, despite an underlying lack of quality. Nevertheless, all else being equal, good teams (those that score many runs and don’t allow many runs) are more likely to make the playoffs and win championships than bad teams.

Estimating Quality
A simple estimate of the quality of your team’s offense is simply the number of runs it has scored so far in the season divided by the number of games it has played (RS/games). Naturally, the accuracy of that estimate will improve as the season progresses and more data is available, but how just quickly does it become accurate? I used Retrosheet game logs from 2000-2013 to examine this question.

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<< Previous Article
Premium Article Pebble Hunting: Martin... (04/28)
<< Previous Column
Moonshot: What PITCHf/... (04/23)
Next Column >>
Moonshot: The Winding ... (05/05)
Next Article >>
Premium Article What You Need to Know:... (04/28)

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