Case Studies

This week class discussion is on the case studies, technology studies and mixed method studies. Regarding case studies some social scientists said, case study does not imply the use of a particular evidence. case studies can be done by using either qualitative or quantitative evidence. The evidence may come from fieldwork, archival records, verbal reports, observations, or any other combination of these. My question is former sentence and later idea  seems to me ambiguous. Again Miles said case studies can be conducted systematically. So why people use this method.