When we want to collect data on a population, there are different ways to proceed. In some cases, it is possible to measure the entire population to get an accurate picture of the nutritional status or mortality rates for the population at hand. This is called an exhaustive survey. These cases are however very rare as they are feasible only for target populations that are geographically concentrated and not exceeding 1000 households, as seen in camps or very small areas. For larger populations, an exhaustive survey will be long, expensive, difficult and unnecessary. In addition, the results of such a survey cannot be extrapolated to other regions.