Researchers undertook a cross sectional study of the use of a coffee creamer as infant food in Laos. To ensure the study was nationally representative, they selected five districts across southern, central, and northern Laos. In total, 84 villages were randomly selected from the five districts. In each village an average of 14 households were randomly selected, and one adult from each household was invited to participate. It was reported that the coffee creamer was often used as a breast milk substitute in Laos, placing the health of infants at risk.1 Which one of the following best describes the sampling method used in the above study?