I did door-to-door data collection for the Canadian census one time, back in the last century (when wooly mammoths still roamed the Earth). Later, I read in our local newspaper (the Victoria Times-Colonist) that the census had been "relatively successful", since we reached 96% of all residents, or some such figure. I was intrigued by this; and I wondered who had counted the 4% that we missed . . . .