Witnesses

There were Jehovah's Witnesses in our street this morning - going door to door to spread the word (can't remember if it's called "the good news" - too long since I was involved). But it got me to thinking, as these things do, about how many houses, units, flats, caravans, huts, rondavels, yurts etc, have not been visited by the Witnesses here and around the world. Is there a different penetration/visitation rate for different countries? Is there a central database somewhere that tracks the visits, callbacks, interest, rude responses? Might be time for a visit to Wikipedia.