In sampling survey we sometimes need facts about sensitive question . attempted open queries about such sensitive issues often result in nonresponse rates or high response biases arising out of willful misstatements or blatant lies 如果对被调查者直接询问，被调查者出于戒备，往往会拒绝回答或不真实回答，这样就破坏了数据的真实性，而且破坏程度的大小无法度量。
