The key word below is aggregator. There has been significant debate on this site regarding the validity of aggregators of various sorts. One could also argue aggregating play by play metrics would by their very nature amplify extremes. http://sabr.org/sdi "The SABR Defensive Index draws on and aggregates two types of existing defensive metrics: those derived from batted ball location-based data and those collected from play-by-play accounts. The three metrics representing batted ball data include Defensive Runs Saved from Baseball Info Solutions, Ultimate Zone Rating developed by noted sabermetrician Mitchel Lichtman, and Runs Effectively Defended based on STATS Zone Rating and built by SABR Defensive Committee member Chris Dial. The two metrics included in the SDI originating from play-by-play data are Defensive Regression Analysis, created by committee member Michael Humphreys, and Total Zone Rating"