An excellent article!
Mr. Nygord enumerates a number of reasons why sight settings might need to be changed, including wind, optical effects and shooter fatigue.
Please note that Mr. Nygord does not advocate chasing shots:
"One does not want to 'chase' each shot, but rather consider the center of the group." He goes on to discuss his definition of a 'group' -- i.e., a cluster of shots that are close enough together to fit within a circle that is the size of the shooter's hold area. This attempts to take into account randomness produced by the limited accuracy of the shooter, gun and ammunition when deciding whether or not the center of a shot group is 'significantly' displaced, so that a sight adjustment is required.
Unfortunately, it fails in this attempt. A shooter's accuracy isn't just defined by the size of his hold area, but also includes other effects, including his skill at operating the trigger. In Mr. Nygord's case, he was a good enough shooter that his hold area was undoubtedly the dominant factor in limiting has accuracy. So, his approach worked for him. For those of us who aren't blessed with Mr. Nygord's level of skill, it won't work so well.
Of course, Don Nygord may also have been defining 'hold area' as the size of the circle that covers 95 percent of your shots, when you shoot a lot of shots over a long period of time. If so, then his approach has greater validity, although it still doesn't account for random 'clumping' of shots.
I agree with you that the thread in question is quite long. So, for a discussion of how to account for shooter accuracy using such concepts as confidence intervals, please see my post:
http://www.targettalk.org/viewtopic.php?p=73352#73352
Although the concept itself is somewhat complex, the calculations use elementary school math. Of course, nobody goes to the range with a calculator. But, on the other hand, it helps to have a realistic understanding of your accuracy when assessing whether or not to change your sights.
I believe that this is what Don Nygord was saying in his article.
Regards,
Al B.