EQA is one of several sabermetric approaches which validated the notion that minor league hitting statistics can be useful measurements of Major League ability. It does this by adjusting a player's raw statistics for park and league.
For instance, the Pacific Coast League is a minor league known to be a very friendly venue for hitters. Therefore, a hitter in the PCL may have notably depressed raw statistics (a lower batting average, fewer home runs, etc.) if he were hitting in another league at the same level. Additionally, in general the level of competition at the PCL is lower than that in the Majors, so a hitter in the PCL would likely have lesser raw statistics in the Majors. EQA is thus useful to strip certain illusions from the surface of players' raw statistics.
EQA is similar to Major League Equivalency (MLE), a metric developed by Bill James.
EQA was used for several years by the authors of the Baseball Prospectus.