The number of samples needed from everyfarming system was calculated as follows:n = (Za S.D.)/L2 in which, n is the sample size, Zathe value of the normal distribution with a confidence a,S.D. the standard deviation, calculated as [P(1 P)]1/2,in which P is the estimated prevalence, L the acceptederror in the final estimation of the prevalence. Thefollowing values were accepted: a = 95%, Za = 1.96and L = 5%. Stratification was based on farming system.