In mechanical terms in the golf swing three variables affect the straightness of shots. Path, contact on face and whether your club face is correct at impact. I believe the path is the foundation of the golf swing. From video taping tour players it is rare that their path would be off neutral (straight to the target ) by more than 2 degrees. In fact one advantage of living in San Diego is a lot of manufacturers are right here in Carlsbad. During the Farmers Insurance Open tour players will go in to get equipment and Trackman data supports the video footage I have gotten over the years.
Every time a student comes for a lesson I check their path not only that it is correct but that it stays the same. If a student normally swings inside two degrees and shows up one degree outside they are still on an acceptable path but that slight change would require a different club face position at impact and could also cause off center contact on the face.
My goal for students is to not miss shots due to a different path. Once you build that into your swing the focus then can be on improving center contact and/or your ability to correctly return the club face at impact.
TEXT GREG