You are referring to a "randomized response" survey[1]. There are also list experiments and endorsement experiments for eliciting responses to sensitive questio

You are referring to a "randomized response" survey[1]. There are also list experiments and endorsement experiments for eliciting responses to sensitive questions[2]. 1: http://en.wikipedia.org/wiki/Randomized_response http://en.wikipedia.org/wiki/Randomized_response 2: http://graemeblair.com/sensitive/ http://graemeblair.com/sensitive/