by John Dewan
Defensive analytics have grown in leaps and bounds in the last decade. At Baseball Info Solutions (BIS), we eat, sleep and breathe defense, but there is always more to learn. A recent research project uncovered some remarkable new information.
One of the public perceptions has been that a player needs three full seasons before his defensive metrics provide a true indication of his defensive abilities. That has been my own personal rule of thumb, though I’ve known there is some reliability to sample sizes smaller than three years.
Based on the new research, BIS has found that Defensive Runs Saved based on as small a sample size as 350 innings in the field (about a quarter of the season) produces reliable results. This is a very significant finding.
The research produced another significant finding. Defensive Runs Saved is a better predictor than many other statistical measures in baseball even over limited samples. Most notably, DRS is a better predictor of future performance than batting average and OPS with partial season data.
We’ll have more on this in the upcoming book, The Fielding Bible—Volume IV, but here is a table that summarizes the results. We use the statistic called the correlation coefficient to show how predictive each statistic is—it produces a number between -1 and 1, with numbers near zero meaning no predictability and numbers near -1 and 1 meaning high predictability.
|Correlation Coefficients of AVG, OPS, and DRS|
|Statistic||350 Innings||700 Innings|
As you can see from the table, DRS is more predictive than batting average and OPS after just 350 innings. The same is true if you increase the samples to 700 innings.
In the study, we ran correlations of three years of defensive data versus the subsequent year’s DRS totals for position players. The first used 350 innings for DRS and 175 at-bats for batting average and OPS—both about one fourth of an MLB season—over both samples. The second used 700 innings and 350 at-bats. The full explanation of the study of the predictive power of Defensive Runs Saved as well as the rest of our latest defensive research can be found in the upcoming Fielding Bible—Volume IV, which will be released in early spring of 2015.