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			| Two or Three Sampling Stages [message #1710] | Tue, 01 April 2014 14:13  |  
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					|  lukassg Messages: 12
 Registered: January 2014
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	| Hey! 
 Could someone explain me exactly what is the difference between three-stage and two-stage sampling?
 
 I very well understand the two-stage sampling:
 
 1. I select a certain number of enumeration areas based on a previous census or national survey.
 2. I sample a certain number of households from these EAs/clusters.
 
 For the three-stage sampling:
 
 As I understand it, this stage is before the EAs/clusters are selected. So we have even larger areas in the first stage, from which we then select the EAs/clusters in the second stage and the households in the third stage? Why do we have this additional stage in the beginning?
 
 Finally, I am pooling always two different DHS survey for one country to get an over-time estimator of certain effects. I know that I have to adapt the stratification and PSU specifications. Is that enough in the case of a country, e.g. Tanzania, where I pool the years 1999 and 2010, where one year has 3 stages and the other 2 stages? Or does that make it impossible to analyse?
 
 In this context, I also stumble upon probability proportional to size (pps). Can someone explain me this concept as well? :)
 
 Thanks for your help!
 
 Lukas
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