The first one is easy - everybody needs to know when they encounter a person who is non-responsive and not breathing normally, and they need to know it well enough that they reflexively call 911 when they see someone who has arrested.
The second question's answer is not so obvious. From the standpoint of getting the job done, we should first train those most likely to witness an arrest.
With about 85 percent of arrests occurring in a private residence, it makes sense to train a population whose age distribution matches that of the victims. Here are the implications of that strategy:
From the CARES database we know that the age distribution of out-of-hospital SCA victims is skewed. We've all known that for quite a while, but the actual numbers are interesting:
We could train more than 80% of the bystanders who are most likely to witness an arrest by only training those who have reached 50!
This also raises the question of what's more important? Training the kids in school or training the faculty with a CPR method they can use from the time a child arrests until the ambulance crew is "hands-on"? (Hint: only one in five can perform what's taught to bystanders for ten minutes.)
Another common - but mistaken - belief is that all it takes to get the chest compressed by two inches is strength. This would be true if you could put one hand on the back and the other on the front of the chest, but few are that strong. Your ability to get to two inches depends upon your weight, your weight distribution, the stiffness of the chest you are trying to compress, and the compression method (hands or heel of foot) you are using.
The one exception is that, if you were taught to "crunch your abs" while performing compression, that boosts the depth of your compression a little bit, but I've never seen anyone be able to do that for more than a few minutes.
Another non-obvious implication is that males and females usually have different weight distributions. Weight below your hips doesn't count at all if you are performing manual compression. Weight at your shoulders really contributes to the force you can apply. Weight half way between your hips and shoulders contributes 50%. If you are performing compression with the heel of your foot, you can get more weight on the victim's chest.