Sometimes, researchers cannot readily locate a list of all members of a population, for example, all people who live in Illinois or all students who attend colleges or universities in the United States. When a study calls for a probability sample from this type of population, a solution is to draw a sample of groups (clusters) from the population of interest for which lists exist or can be developed. A sample of members of each group can then be drawn in the next stage. The result is called a multistage cluster sample.