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Sample Size for Surveys - Objectives of the Survey 

The objectives of the study can be:  a- To estimate a parameter prevalence, an Odds ratio, etc.  b- To test an hypothesis by comparing two( or more ) groups.  c- To test if a Prevalence, Odds Ratio or Relative Risk is significant. ie between Exposed and non exposed, A 'clean ' city versus a 'polluted' city, etc. For each objective the sample size needed has different requirements.