Gene McKenna, a friend of mine, who is a very good player contributed to my blog with this important pointer:
“Considering that the vast majority of amateurs hit the ball with an outside to inside swing path, then it goes without saying that the “typical” slice is hit with an outside to inside swing path. The slice is by far the most common shot among amateurs. By far. OK, some amateurs hit their shots straight, and some hook the ball, and some pull it and some push it, but it is an extremely common shot for amateurs to hit a slice. (And if we are talking about fades as well, well, a fade is more or less just a “baby” slice that typically is well controlled and is also often quite intentional.) What percentage of all golfers are pros? Maybe around one-percent or less would be my guess. In any case, it is a very small percentage. There is no way that the percentage could be more than the low single digits, 3% or 4% absolute maximum. Therefore the vast majority of golfers are amateur golfers, and, as this article makes clear, the “vast majority of amateurs” have out to in swing paths. It also kind of goes without saying that they “typical” pro golfer does not use an outside to inside swing path, but rather uses an “inside to outside” (AKA “in to out”) swing path. This typically gives more power, and it is also typcially used for most draw shots if / when a pro wants to hit a draw”
Read the article (click on the word article). It has a drill to force an in to out swing path.