ISO (Isolated Power) measures a hitter's pure power output by stripping batting average out of slugging. The formula: ISO = SLG - AVG. Elite is .200+; league average is around .160. ISO is the cleanest single-stat power indicator.
Slugging Percentage counts total bases per at-bat. But slugging includes singles, which aren't extra-base power. ISO subtracts batting average from slugging, leaving only the EXTRA bases (doubles, triples, HRs) per AB. The formula: ISO = SLG - AVG equivalently ISO = (2B + 2×3B + 3×HR) / AB.
The result is pure power isolation. A hitter with .280 AVG and .500 SLG has .220 ISO. A hitter with .200 AVG and .500 SLG also has .300 ISO — same power output, less contact.
ISO is the cleanest predictor of HR rate, which matters for:
SLG includes singles, so a slap hitter who never strikes out can have high SLG without much power. ISO isolates the power component. For evaluating REAL power threat, ISO is strictly better than SLG.
ISO is highly park-dependent. Hitters at HR-friendly parks (Coors, Great American, Yankee Stadium, Citizens Bank) post inflated ISOs that don't travel to road games. Hitters at HR-suppressing parks (Petco, Marlins Park, Oakland) post deflated ISOs at home but real power on the road. For predictive use, look at road ISO separately or use park-adjusted ISO from FanGraphs.
Most hitters have meaningful ISO splits vs LHP vs RHP. A LHB power hitter might post .280 ISO vs RHP but .150 ISO vs LHP. Lineup ISO vs the opposing starter's handedness is the predictive number for tonight's game, not season-overall ISO.
Lineup ISO vs handedness is one of the model's primary offensive inputs for HR and run-scoring estimates. Combined with park HR factor and pitcher HR/9 (also handedness-split), it produces the per-at-bat HR probability that feeds total run expectation and YRFI probability.
Theory is one thing — using the concept on tonight's slate is the value. These hubs are the practical application: