Double Naught Spy got it right in post #8, with one exception.
The cause of better than 80% of hip fractures in the elderly is indeed in the acetabulum or the trochanter of the femur, weakened through osteoporosis. What commonly occurs is the elderly person falls. The weakest part of the hip joint (whether trochanter or acetabulum) is compromised through a partial fracture. Either the elderly person themself tries to stand, or someone else tries to help them up. The elderly person must put weight on the joint to stand. The weight completes the process, and you have a total fracture. The elderly person often lies about the fall if they were alone, because they are scared of nursing home placement. The bruises tell the story though. Occasionally the same area will fracture due to additional weight the person tries to carry, or a movement that places undue stress on the joint. Moral is: Leave them on the floor and call the paramedics. Have the paramedics use a "scoop" or "clamshell" to get them off the floor. The recovery time from a partial fracture will be much less.
I shoot COM, and I know anatomy pretty well from my work. I will shoot COM while seeking cover. For me, it's a matter of tactics. I'm shooting at a moving target while I'm moving myself, and I'm under considerable stress. A larger target yeilds a higher probability of a hit. COM is the larger target. It has a lot of stuff that can slow down the BG. It is enclosed by bones about 3/8 thick, and exposed between them. There is a high probability of significant penetration. If I feel I am down to half capacity with a threat still coming, I might try for a head shot if it presents a good option. If it is my only option, I'm taking it.
I'm of the opinion that "pelvic shots" are an attempt to rewrite a book that is already written well. Trainers have to improve their product to keep customers coming back. One thing about the "pelvic shot" is exactly what has been discussed here. It's a small target. It also moves quickly, and it can be obstructed by the ilium from a partially frontal shot. I don't think I would ever try a shot to this area. If I had to take a shot to the pelvis because none other was available, I would try for the pubis. This is the pelvic bone that crosses in front of the pelvis. It has a permanent joint in the center and can be quite fragile there. If you feel your belly directly above your 'nads, you will feel this bone about 2 and a half inches in (or 4 inches in some folks
) This area is the support structure for all the inner tendons of both legs. Shatter it, and the person is no longer walking. The problem again is the target size. You are talking about a 1 inch diameter area on a moving target. It could easily be a waste of firepower. If for some reason, however, I could not get a COM or head shot, and I had a frontal below the waist selection available, This is the area I would shoot.
The most important aspect of survival (at least for Joe Civilan) is to get the hell out of there, or finding cover if you cannot. That means moving. Moving is more important than shooting IMHO. Your movement makes you a much more difficult target, especially if you are moving laterally. It effectively neutralizes the BG's ability and makes him rely on luck. Being able to shoot accurately on the move makes you a very dangerous person to be shooting at.
The next time you are at the range take a milk jug, 50 feet of string and a stake. Tie the string to the jug, set the stake out about 20 feet and place the jug to the side about 20 feet away from the stake. Place the string around the stake and back to your shooting postion, in effect forming a right angle from above. Now have a buddy pull the jug laterally across your field of fire while you try to hit it. When you get to where you can hit it consistently, try it while moving yourself in another direction. Try it one handed. Do not stop to shoot. Keep moving. It will quickly become apparent how inadequate target shooting is for self defense shooting.