This reminds me of another technique I learned in a PoliSci class (too many years ago). The prof had been polling people about sensitive topics like race. His survey asked respondents only yes/no questions and asked them only to listen to each question and privately write down yes or no to each question. Then they were asked to count the yeses. By only asking 1/2 the survey group the sensitive question, you get a measure of the true ratio for the sensitive question. You end up doubling the sample size to get the same number of responses, but you allow people to hide behind their other answers.