I met Dad at work in DC one time for lunch, and as we were walking to the restaurant we saw a guy with a sign that said something like "Homeless and Hungry Vietnam Vet, Please Help." After lunch, Dad decided to do something so we stopped by McDonald's and bought a meal deal (don't remember which one, it's immaterial) and walked back by the guy. Dad handed him the bag and drink and said something to the effect of "I just wanted to help out a fellow Vietnam Vet." The guy sneered at us, and threw it in the trash can.
Because of that experience, I don't put much stock in the "statistics." Was he hungry? Maybe - but more likely hungry for booze. Was he a Vietnam Vet? We'll never know, but his credibility was destroyed at that moment.
Also because of that experience, I generally don't give money to homeless on the streets. I would prefer to give money to a legitmate organization that is trying to help. I did give a guy $5 one time though - because he was standing there with a sign that said "Homeless Alcoholic, I need a beer!", and told him that I admired his honesty.