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Sometimes you do not want every element of the population to have an equal chance of being selected. Sometimes, you want to find disease, if it is there. Surveys like this are called "risk-based", because high-risk sites or individuals are more likely to be sampled than are low-risk ones. These types of surveys cannot provide unbiased estimates of prevalence. They can also provide good evidence for disease freedom. The planning and analysis of risk-based surveys needs expert advice.