[ngkabra]

This description is a bit confusing, so I'll assume that the protocol was this:

100 people are asked a possibly self-incriminating question - "Are you indulging in bad activity X?" They are expected to toss a coin. If the coin said tails, they are expected to give a truthful answer. If the coin said heads, they are expected to say "Yes."

Now, no single person needs to worry about self-incrimination, since they can always claim that their "Yes" answer was the result of a "heads" on the coin toss. But still, you can get an estimate of the percentage of bad activity X going on, by subtracting 50 from the number of "Yes" answers and doubling.

(This is assuming of course that the subjects believe your claim that there is no way for individuals to self-incriminate - which is certainly not a given, IMO.)

[ams6110]

Yeah, I think you still have to assume your "yes" count is going to be somewhat low because some number of subjects will not trust you regardless of the coin-flip cover.