Fig. 3.

Detailed description example, for sample size calculation and randomization processes for repeated surveys. PriorTP = The true prevalence (TP) assumed before the survey is made. Se = test sensitivity. Sp = test specificity. PIntro = Probability of introduction into the targeted population Np (e.g. during 1 or more sampling days or during time elapsed between two consecutive surveys). PFree = The aimed confidence in freedom to be substantiated by the survey, if all samples (n2) result negative (confidence TP < Pu); Np = The size of the population within the targeted area and considered for the randomization process. Pu = The design prevalence of infected people, at which we can have at least one positive result out of n2 sampled, with the test used. N.B. If both n1 and n2 are calculated, by using the biggest of the two sample sizes, would prepare for both outputs, namely TP (if at least one person is positive) and PFree if all sampled persons result negative. Also notice that, usually, the specificity (Sp) is assumed 100% [[4], [5]] when PFree is assessed.