What is strokes gained in golf?

Definition

Here's how it works: Every shot you hit is assigned an expected number of strokes needed to hole out from that position based on the distance to the hole and the lie you're hitting from. Those expected strokes vary depending on the level of golfer you're benchmarking against (i.e., a PGA Tour player is expected to need less strokes from 160 yards away then a 15 handicap). If you leave your golf shot in a better position than expected, you gain strokes. If you leave it in a worse position, you lose strokes. For example, let's say you're a 15 handicap golfer and you're 200 yards out in the fairway. A PGA Tour pro is expected to take 3.16 strokes to hole out from that distance. You knock it to 20ft - a position where, on average, a Tour Pro will take 1.87 shots to finish the hole. In this case, the formula would be [3.16 - 1.87 - 1 = 0.29] - you gained 0.29 strokes in that shot vs a Tour Pro (the "1" is the shot you took). If you pull it left, and leave it 30 yards away on the rough, you're going to lose strokes because, from there, on average, a Tour Pro will take 2.61 strokes to finish the hole: [3.16 - 2.61 - 1 = -0.45] - you lost 0.45 strokes in that shot vs a Tour Pro.