Well, I don't have a back door. I have a slider on one side of the patio from the family room, and a slider on the other side of the patio that go into the backyard, but no back door. Since there is no lock on the outside of the sliders, if the door is locked you can't get in from the back. My parents house had a slider in the family room and a slider from the living room into the backyard and they had a back door, with it's own door bell*, out of the laundry room into the backyard. I say had, because I sold it to investors who remodeled it, took out the back door, and split the laundry room into thirds. One third has a stacked washer and dryer, one third was turned into a walk in closet for the master bathroom, and the master bath was extended into the final third.
*The front door door bell make a ding dong sound, the back door bell sound was just the ding. House was built in 1960 when home delivery of dairy products was common, so the milk man would leave your order on the back step, ring the bell and leave.