Supplement 2:

1. Supplement 2: Asking Sensitive Questions in Surveys

2. “Sex can wait until we are married, say youngsters”2007-02-12 HK Standard • Young people say sex before marriage is not widely acceptable, although it is perceived to be so, a survey has found. • The Hong Kong Sex Culture Society, a Christian group, polled 943 students from tertiary institutions and found that 51.3 percent felt that pre-marital sexual intercourse is acceptable, but the remainder disagreed. • Nearly 30 percent of those interviewed said they would have a problem if their life-long partners were not virgins before marriage and another 41 percent said otherwise. • Despite the divergence of attitudes to sex, 411 students said kissing is the most commonly acceptable form of intimate behavior, while 27.4 percent preferred sexual intercourse, followed by 13.6 percent who opted for caressing. • According to the poll, students with a religious background are sexually more conservative and have stronger faith in life-long marriages. • More than half the students, or 448, who were mostly polled through Internet questionnaires and a small portion by street interviews, are religious, with 434 of them being Christians.

3. Examples of sensitive questions • Have you ever used illegal drugs? • Have you ever had an abortion? • Have you had more than one sexual partner in the past 6 months? • Have you ever driven a motor vehicle while intoxicated? • Have you ever shoplifted? • Have you cheated in exam in the past six months? • Have you ever attempted suicide? • Have you ever invaded tax?

4. Asking Sensitive Questions in Surveys • How to frame a question so that the respondents have no incentive to lie? • Only if your answer to the survey question cannot be used to infer your true answer to the intended question.

5. The survey question • Spin a coin twice. Show no one the results. If the first spin resulted in a head, answer the question marked H. If the first spin resulted in a tail, answer the question marked T. • H: Did you have one or more sexual partners in the past 12 months? • T: Did the second spin of the coin result in a tail? The respondent has no incentive to lie because only the respondent knows which question he / she is responding to.

6. The calculations • The respondent will answer question H with probability q and T with probability (1-q). If the coin is fair, q=0.5. • Let the known probability of a YES to question T be r. If the coin is fair, r=0.5. • Let p be the proportion of students who have one or more sexual partners in the past 12 months. That is, those who answer YES to question H. • So, the probability of answering YES to the question is Prob(YES) =q*p + (1-q)*r • Hence, p = [ Prob(YES) –(1-q)*r ] / q.

7. Did you pay tax? • According to a survey done by the National Bureau of Statistics of China in 2002 (covering about 700 residents of Beijing, Shanghai and Guangzhou): Source: http://www.stats.gov.cn/was40/reldetail.jsp?docid=21970 薪酬系列调查之二：个税调节力度不够

8. References: • “Randomized Response” by Donald Bentley of Pomona College provide a good summary of the technical issues, as well as a list of classic references on the topic.(http://www.dartmouth.edu/~chance/teaching_aids/RResponse/RResponse.html)

9. Supplement 2: Asking Sensitive Questions in Surveys - END -