Will a dog protect me?
Dogs are wonderful companions, large ones are intimidating (to potential perpetrators) outdoors and in cars, loud ones a deterrent from behind a locked door, and most an excellent early warning system.
Their superior hearing allows them to detect and warn of intruders long before humans might notice (a "furry alarm system").
Dogs can discourage potential attackers merely by their presence. Interviews with career criminals indicate that dogs are a factor in their selection of victims. Several women have reported people actively avoiding them on the street when accompanied by large dogs.
Dogs can actively participate in the defense of their human companions, but not all dogs do this well. This may either be instinctive or trained. Training requires a substantial investment, both for the dog and the owner.
If you have a dog, you should not count on it as your primary, or only, method of defense.