many subjects don’t give honest answers to questions about activities that are illegal or sensitive in some other way. one study divided a large group of white adults into thirds at random. all were asked if they had ever used cocaine. the first group was interviewed by telephone: 21% said "yes." in the group visited at home by an interviewer, 25% said "yes." the final group was interviewed at home but answered the question on an anonymous form that they sealed in an envelope. of this group, 28% said they had used cocaine.