Fielding independent pitching

In baseball, defense-independent pitching statistics (DIPS) measure a pitcher's effectiveness based only on statistics that do not involve fielders (except the catcher). These include home runs allowed, strikeouts, hit batters, walks, and, more recently, fly ball percentage, ground ball percentage, and (to much a lesser extent) line drive percentage. By focusing on these statistics, which the pitcher has almost total control over, and ignoring what happens once a ball is put in play, which the pitcher has little control over, DIPS can offer a clearer picture of the pitcher's true ability.

Originally, the most controversial part of DIPS was the idea that pitchers have little influence over what happens to balls that are put into play. But this has since been well established (see below), primarily by showing the large variability of most pitchers' BABIP from year to year. In fact, the outcome of balls in play is dictated largely by the quality and/or arrangement of the defense behind the pitcher, and by a good deal of luck. For example, an outfielder may make an exceptionally strong diving catch to prevent a hit, or a base runner may beat a play to a base on a ball thrown from a fielder with sub-par arm strength.

In 1999, Voros McCracken became the first to detail and publicize these effects to the baseball research community when he wrote on rec.sport.baseball, "I've been working on a pitching evaluation tool and thought I'd post it here to get some feedback. I call it 'Defensive Independent Pitching' and what it does is evaluate a pitcher base[d] strictly on the statistics his defense has no ability to affect. . ." . Until the publication of a more widely read article in 2001, however, on Baseball Prospectus, most of the baseball research community believed that individual pitchers had an inherent ability to prevent hits on balls in play. McCracken reasoned that if this ability existed, it would be noticeable in a pitcher's 'Batting Average on Balls In Play' (BABIP). His research found the opposite to be true: that while a pitcher's ability to cause strikeouts or prevent home runs remained somewhat constant from season to season, his ability to prevent hits on balls in play did not.

...
Wikipedia