We wish to survey n people to estimate the proportion p of all people who have lied on an survey. People are asked to ?ip 2 coins. If both times it comes up as heads, they are asked to reply "no" if they have lied and "yes"if they have not. Otherwise, they are asked to reply "yes" if they have lied and "no"if they have not. Suppose X people answer "yes". Show that (if everyone tells the truth), phat = (2X/n) - (1/2) is an unbiased estimator of p.