A good solid Martial Arts training can reduce attacks, experience shows that most of the people with solid martial arts training desire to AVOID violence and fights. They feel comfortable in themselves and don't feel they have to "prove" anything and thus are more willing to walk away or find a non-violent means. Also in their training they have been hit and know it hurts!
An effective self defense class should include as the number one topic
"HOW TO : NOT GET IN TROUBLE IN THE FIRST PLACE".
What to look out for(areas, traffic flow, surroundings, etc.) so you can avoid the encounter. (or at least reduce probability of having one.) If one can avoid the situation that is a 100 times better than "trying to fight it out". In addition, even if you have a lot of training and are a "lethal killing machine" then do you really want to use it? Now you have the legal problem of "deadly force"; what do you do if you have effectively subdued the attacker and now you are arrested for killing or seriously injuring the attacker? Better to avoid the whole thing!