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Sample Size Newsbeat

Monday, June 22, 2015

A New Way to Look at Sample Size

Our aim in this project is to extend the understanding of reliability and show a more complete picture of how additional plate appearances affect the reliability value of everyday stats for both batters and pitchers. We want to reinforce the idea that reliability is a spectrum, not a single point. There is no single point at which you can say a stat has stabilized. We also want to use the concept of reliability to regress toward the mean and to make confidence bands that give a better idea of a player’s true talent. (Throughout this project we define true talent as the actual talent level — not the value they provide adjusted to park, competition, etc.)....

The most important conclusion to be drawn is there is no single point at which a stat becomes reliable or stable. The alpha reliability data visualization demonstrates that idea, using the reliability measurement to regress the stat to the mean and create confidence bands. The regressed stat and confidence bands are descriptive, rather than predictive, and are not adjusted for park factors, league adjustments, etc. This can provide an estimation of a player’s true-talent level based on how the player has performed. They aren’t intended to be projections.

RoyalsRetro (AG#1F) Posted: June 22, 2015 at 11:36 AM | 1 comment(s)
  Beats: sample size

Sunday, April 19, 2015

The Economist: One swing is all you need

Why one moonshot can matter more than years of past performance: applying Bill James’s concept of “signature significance” to home run distance.

The group of 22 players who have hit at least one 477-foot homer over the past eight seasons…features Mike Trout…Giancarlo Stanton…Adam Dunn, Prince Fielder, Josh Hamilton, David Ortiz, Nelson Cruz, Justin Upton, Edwin Encarnación and Mark Teixeira…

For each foot beyond the distance of a league-average longball (usually just under 400 feet) that any individual home run travels, an additional 0.06% of that batter’s other line drives and outfield fly balls in that season become home runs…If a player about whom we had no other information—say, a recent arrival from Cuba—hit a home run of average distance in his first at-bat, we would expect him to hit about 6% more homers than an average player for the rest of the season. In contrast, if the player hit a 477-foot home run like Mr Rodríguez’s in his first at-bat, we would expect him to hit around 50% more homers than average…

The fact that [Alex] Rodríguez propelled a single baseball 477 feet means there is a very strong chance he is not the player we thought he was. Guys who are washed up just don’t hit 477-foot homers. Not even once.



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