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
Sites have to assume uniform system with regard to issue.