Explanatory Value of Exit Velocity Data

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Posted on Feb 20, 2018

Overview

Beginning with the 2015 season, Major League Baseball began capturing and publishing Statcast data, which uses state of the art cameras to capture and quantify previously unquantifiable baseball activity, such as the positioning and range of fielders, and the speed (exit velocity) and angle (launch angle) at which the baseball leaves the hitter's bat. As there are now three seasons of available Statcast data, I set out to explore what explanatory power exit velocity has with respect to established measures of power and run production.

Data Capture

I used Python's Scrapy library to marry Fangraph's Advanced Batter Statistics and Batted Ball data to the Statcast Exit Velocity data for all players with more than 30 at bats for the 2015, 2016, and 2017 seasons. I filtered the exit velocity data for line drives and fly balls; popups and ground balls theoretically should have little to no bearing on power production. Statcast data is extremely granular, individual plays are available. For this exercise however, I only looked at season averages as a jumping off point. I made this choice largely because Statcast frowns upon scraping, and makes the most granular data available primarily via CSV export. This analysis was undertaken under the guise of our Web Scraping project, exporting CSV data would somewhat defeat the purpose. I intend to incorporate more granular data in future analyses. For this analysis I was able to retrieve a sample of 310 player seasons using a recursive Scrapy spider.

Regressions

My initial hypothesis was that Exit Velocity on line drives and fly balls would be a statistically significant predictor of a batter's power production. (The corresponding null hypothesis was that there would be no statistical significance to Exit Velocity as an explanatory variable). I ran linear regressions of ISO (isolated power), and Home Run to Fly Ball Ratio against Exit Velocity.

ISO is calculated as SLG (Slugging percentage: total bases/plate appearances) - OBP (On Base Percentage: times on base/plate appearances); ISO attempts to measure power production by eliminating the reaching base component from offensive output, you can think of it as extra bases per plate appearance. HR/FB ratio is the percentage of balls hit in the air that become Home Runs.

The rationale here is that logically, the faster the ball travels, the more likely it is to leave the park or land for an extra base hit. Velocity is not the sole determinant of outcome, angle, direction relative to fielder positioning, dimensions of the park, wind speed and direction are some of the other variables expected to influence offensive outcomes. Therefore the expectation would be statistical significance but not an overly large coefficient of determination, as these other variables are not accounted for.

As anticipated, the simple linear regressions produced large t-values (19.6 and 16.1) indicating statistical significance with greater than 99.9% confidence, but moderate coefficients of determination (0.555 and 0.456) for HR/FB and ISO respectively. These results disprove the null hypothesis and support the expectations.

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