Also, it occurs to me that this is essentially an application of Bayes' Theorem. In an ordinary survey, the posterior probability (killed leopard|says yes) is 1, which is bad for the farmers, so they lie and therefore decrease the conditional probability (says yes|killed leopard), which is bad for the surveyors. Adding the die roll increases the unconditional probability of saying yes, so that the posterior probability no longer equals the conditional, and they can both get what they want.