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If a disease is rare, or if the "gold standard" test is complex and expensive to perform, sample sizes for estimation of sensitivity are likely to be small, leading to imprecise estimates of sensitivity. If a disease does not occur in a country it is impossible to estimate sensitivity in a sample that is representative of the population in which it is to be applied. Conversely, if a disease does not occur in a country or region, it is relatively easy to estimate test specificity, based on a representative sample of animals from the population, because if the population is free of disease all animals in the population must also be disease-free.