I agree with Rod. Most people have true focus for only about the 3 seconds - it makes our focus much like Goldfish.
To help prove the concept to non-believers, we have a large clock and get about 5 students to clap on the second for 5 seconds. Then we move to claps every five seconds, then 30 and 60 seconds. You'll notice that everyone is in sink at the shorter durations, but as the time distances get longer the claps get spaced further.
Problem is though is getting people who suffer with ADD to focus or concentrate at all without distraction. Then once they get the concentration to keep it consistent from shot to shot. The ultimate challenge is to get them to have a repeatable, consistent thought process throughout the shot cycle.
What we try to do is to ultimately associate counting with steps in the shot sequence. We usually use a three count to hold (1 draw, 2 anchor 3 transfer), and then a three count through expansion (there is also some stuff before the draw). Aiming only takes place in expansion which is another three count. There is one final three count during follow through. It's almost like a waltz, 1-2-3, 1-2-3. Even if the count stops at expansion, subconscious timing can be set for the shot. The draw (count one) sets the speed of the shot sequence. If there is a quick draw then there is a quick anchor, etc. The counting is done at the conscious level at first but gets imbedded.
In order to actually evaluate the focus/concentration, we use NeuroSky technology which is able to show levels of concentration and meditation during the shot sequence. Focus training and relaxation can also be practiced on the NeuroSky system.
Other focus training for example, can be done, such as having a random order number grid. The individual is asked to find five number sequences with the starting number given by the timer. As focus increases, time decreases.