The survey sampler does not always have a list of all the members of a population. In cluster sampling, the population is organized in groups called clusters. Clusters often occur naturally; examples include persons in the same household, residents of the same county, birds in the same nest, or patients in the same hospital. In cluster sampling, clusters are randomly selected for the sample. Then, either all individuals in the sampled clusters are measured, or a sample of individuals is selected from each sampled cluster. Because individuals within the same cluster often share environmental factors, they tend to be more similar than individuals in different clusters. The methods for cluster sampling specify how to design a cluster sample and how to account for that similarity when estimating variances.