text: Site selects itself in specific problem-for decisive testing , have to assume uniform system with regard to issue and so convenience sample acceptable ; number of cases is usually one instance ; comprehensive data for specific problem-for decisive testing , need more modeling , hypotheses , and targeting to know what to study ; data analysis and collection concurrent and data feed new collection , and emphasis on ruling out alternative causes ; report describes instances , presents conclusions about cause , gives evidence
  
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  Number of cases is always one instance .