1. Flip a coin. If it comes up heads, answer the "heads " question. If comes up tails, answer the "tails " question. Put your answer (a or b) here _______.
Heads question: Are you a college student?
Tails question: Have you used cocaine in the last two months?
2. Suppose we give this survey to 24 college students. How many will get the "heads " question? ____
3. How will they answer? ____ Are we interested in their responses?
4. Given that we are not interested in their answers, we can subtract out their responses. Thus, if we received a total of 15 "a" responses, we would subtract 12 from 15, leaving us with 3 people who received the tails question and answered "a."
5. We can now assume that approximately 3 people who got the tails question answered "a. " To get the percentage of people who got the tails question and answered "a, "we divide 3 by 12 (the number of people who probably got the tails question). Thus, we estimate that 3/12 or 25% of the people who got the tails question answered "a. "
6. Since there is no reason to assume that the people who got the "tails " question are any different from those who got the "heads " question, we would assume that the percentage of the 24 students who were cocaine users was approximately 25%.