Editing Cluster sampling (section)

===Two-stage cluster sampling===
Two-stage cluster sampling, a simple case of [[multistage sampling]], is obtained by selecting cluster samples in the first stage and then selecting a sample of elements from every sampled cluster. Consider a population of ''N'' clusters in total. In the first stage, ''n'' clusters are selected using the ordinary cluster sampling method. In the second stage, [[simple random sampling]] is usually used.<ref>{{cite book|last=Ahmed|first=Saifuddin|title=Methods in Sample Surveys|year=2009|publisher=The Johns Hopkins University and Saifuddin Ahmed|url=http://ocw.jhsph.edu/courses/statmethodsforsamplesurveys/PDFs/Lecture5.pdf |archive-url=https://web.archive.org/web/20130928180152/http://ocw.jhsph.edu/courses/statmethodsforsamplesurveys/PDFs/Lecture5.pdf |archive-date=2013-09-28 |url-status=live}}</ref> It is used separately in every cluster and the numbers of elements selected from different clusters are not necessarily equal. The total number of clusters ''N'', the number of clusters selected ''n'', and the numbers of elements from selected clusters need to be pre-determined by the survey designer. Two-stage cluster sampling aims at minimizing survey costs and at the same time controlling the uncertainty related to estimates of interest.<ref>{{cite book| author = Daniel Pfeffermann|author2=C. Radhakrishna Rao| title = Handbook of Statistics Vol.29A Sample Surveys: Theory, Methods and Infernece| url = https://books.google.com/books?id=waHxMgEACAAJ| year = 2009| publisher = Elsevier B.V.| isbn = 978-0-444-53124-7 }}</ref> This method can be used in health and social sciences. For instance, researchers used two-stage cluster sampling to generate a representative sample of the Iraqi population to conduct mortality surveys.<ref>{{cite journal|author=LP Galway |author2=Nathaniel Bell |author3=Al S SAE |author4=Amy Hagopian |author5=Gilbert Burnham |author6=Abraham Flaxman |author7=Wiliam M Weiss |author8=Julie Rajaratnam |author9=Tim K Takaro|title=A two-stage cluster sampling method using gridded population data, a GIS, and Google EarthTM imagery in a population-based mortality survey in Iraq|journal=International Journal of Health Geographics|volume=11 |pages=12 |date=27 April 2012|issue=1 |doi=10.1186/1476-072X-11-12 |pmid=22540266 |pmc=3490933 |doi-access=free |bibcode=2012IJHGg..11...12G }}</ref> Sampling in this method can be quicker and more reliable than other methods, which is why this method is now used frequently.