I understand why we are discussing this, but the number of random knocks at a door by persons unknown to the occupants who have a nefarious intent is similar to the number of little leaguers in your town who will one day play in the World Series. Of course, there are many psycho little league and high school baseball parents who believe otherwise . . .
Added: My count for both remains at zero . . .
Depends where you live. Super-rural, probly pretty low. Suburbs and denser? Higher than you'd think.
Not everyone nefarious is going to do something right then. Most of the time, they're using the door-knock to case prospective targets. They want to know who's home, how many people, what the response is like, good stuff visible, etc.
Hitting the house right then makes no sense, because if somebody's home, they'll have been seen. But casing that way goes pretty much unnoticed.